Cerebroplacental Ratio/CPR

UCA Doppler study

Cerebroplacental Ratio (CPR) & normal value

Normal Value

Cerebroplacental Ratio (CPR) Fetal Sonography Updated 2026 Educational Reference

CPR (Cerebroplacental Ratio) →
Cerebroplacental Ratio (CPR) is the ratio of the pulsatility index (PI) of the middle cerebral artery (MCA) to the pulsatility index (PI) of the umbilical artery (UA). It is a composite Doppler index that compares cerebral vascular resistance with placental vascular resistance, providing a single number that reflects the balance of blood flow distribution between the fetal brain and the placenta.

Formula:
  CPR = MCA PI ÷ UA PI

What it measures:
CPR assesses whether the fetus is preferentially shunting blood toward the brain at the expense of the placenta — a phenomenon known as "brain-sparing".

In a healthy fetus with a well-functioning placenta:
  • The UA has low resistance (low UA PI) → blood flows easily to the placenta.
  • The MCA has moderate-to-high resistance (higher MCA PI) → the brain does not need to dilate its vessels because oxygenation is adequate.
  • Therefore CPR is > 1.0 (MCA PI > UA PI) — this is normal.

In a fetus with placental insufficiency and hypoxia:
  • The UA has high resistance (high UA PI) → reduced placental flow.
  • The MCA dilates to increase cerebral blood flow (low MCA PI) → the brain attempts to protect itself from hypoxia.
  • Therefore CPR falls below 1.0 (MCA PI < UA PI) — this is brain-sparing and indicates fetal compromise.

Why CPR is important:
  • CPR can become abnormal before either the UA PI or MCA PI alone crosses their respective 95th or 5th centile individually.
  • It detects the redistribution phase of fetal adaptation to hypoxia — the brain dilates before the UA resistance becomes critically high.
  • A low CPR is associated with adverse perinatal outcomes including IUGR, neonatal intensive care unit (NICU) admission, low Apgar scores, and long-term neurodevelopmental impairment.
  • CPR is more sensitive than UA Doppler alone for detecting early fetal compromise, especially in late-onset FGR (fetal growth restriction).
  • In late-onset FGR (after 32 weeks), UA Doppler is frequently normal, but CPR may be the only abnormal finding.

Normal physiological trend:
CPR is relatively stable across gestation but declines slightly with advancing gestational age. This is because both MCA PI and UA PI fall with gestation, but in late pregnancy the MCA PI falls faster than the UA PI in some fetuses, causing a natural decline in CPR near term. However, CPR should remain above 1.0 throughout gestation in a healthy fetus.

  • 20–28 weeks: CPR typically 1.5–2.5
  • 28–34 weeks: CPR typically 1.3–2.0
  • 34–40 weeks: CPR typically 1.1–1.8
  • At term (40 weeks): CPR may approach ~1.0–1.2 in normal fetuses, but should not fall below 1.0

Cut-off values:
There are two commonly used thresholds for defining abnormal CPR:

  1. Fixed cut-off:
    CPR < 1.0 is considered abnormal at any gestational age.
    Simple and easy to remember, but less precise because it does not account for the natural gestational-age-related decline in CPR.

  2. Centile-based cut-off (preferred):
    CPR < 5th centile (or < 10th centile) for the gestational age.
    More accurate — accounts for the gestational-age-related decline in CPR.
    This is the recommended method by most guidelines (ISUOG, Delphi consensus).

  3. Multiple of the Median (MoM):
    CPR < 0.6765 MoM is considered abnormal.
    Used in some research settings and reference charts.

CPR in the context of fetal deterioration:
In fetal growth restriction (FGR), the Doppler deterioration sequence typically follows this order:

    1. CPR falls (brain-sparing begins — MCA dilates, UA PI may still be normal)
       ↓
    2. MCA PI falls below 5th centile (cerebral vasodilation overt)
       ↓
    3. UA PI rises above 95th centile (placental resistance rising)
       ↓
    4. UA AEDF (absent end-diastolic flow)
       ↓
    5. UA REDF (reversed end-diastolic flow)
       ↓
    6. MCA PI normalises / rises (brain-sparing lost — decompensation; brain can no longer maintain vasodilation)
       ↓
    7. Abnormal CTG / BPP (overt fetal distress)

  Key point: CPR is the earliest Doppler marker to become abnormal in FGR — it changes before UA PI, MCA PI alone, or CTG.

Early-onset vs late-onset FGR:
  • Early-onset FGR (<32 weeks): Placental insufficiency is severe. UA Doppler is usually abnormal (elevated PI, AEDF, REDF). CPR is low. MCA PI is low. The sequence is: CPR ↓ → UA PI ↑ → AEDF → REDF → MCA normalisation → CTG abnormal.
  • Late-onset FGR (≥32 weeks): Placental insufficiency is milder and more subtle. UA Doppler is frequently normal. The only abnormal finding may be a low CPR (with or without low MCA PI). This makes CPR essential in late-onset FGR screening.

Measuring technique for CPR:
CPR requires two separate Doppler measurements — the MCA PI and the UA PI — and then their ratio is calculated.

  Step 1: Measure UA PI
  1. Identify a free-floating loop of umbilical cord.
  2. Apply color Doppler and place PW sample volume (2–3 mm) over the artery.
  3. Angle of insonation close to 0° (angle correction not needed for PI).
  4. Capture ≥3 uniform waveforms during fetal rest.
  5. Record the mean UA PI.

  Step 2: Measure MCA PI
  1. Obtain an axial transverse plane of the fetal head at the level of the thalami and cavum septum pellucidum.
  2. Activate color Doppler to visualize the Circle of Willis.
  3. Identify the middle cerebral artery — the largest lateral branch of the Circle of Willis, running anterolaterally from the internal carotid artery.
  4. Place the PW sample volume (2–3 mm) over the proximal third of the MCA, within 2 mm of its origin from the internal carotid artery (avoiding the distal segments where resistance is higher).
  5. Angle of insonation close to 0° — ideally insonate the portion of the MCA where the vessel runs toward or away from the probe. PI is angle-independent, but a good angle ensures a clean waveform.
  6. Ensure the fetus is in a quiet resting state (no breathing, no movement). Ideally measure when the fetus is not actively moving.
  7. Avoid measuring during fetal breathing — it alters venous return and affects cerebral flow.
  8. Capture ≥3 uniform waveforms.
  9. Record the mean MCA PI.

  Step 3: Calculate CPR
  CPR = MCA PI ÷ UA PI
  Most ultrasound machines auto-calculate CPR when both PI values are entered.

  Important technical notes for MCA:
  • Do not apply angle correction for MCA PI — PI is a ratio and is angle-independent.
  • Measure the proximal third of the MCA — distal segments have higher resistance and will falsely lower MCA PI (and thus falsely lower CPR).
  • Do not press the probe hard on the maternal abdomen — excessive pressure can alter fetal intracranial pressure and falsely elevate MCA PI.
  • Ensure the fetal head is not deeply engaged in the pelvis — a deeply engaged head can compress the MCA and falsely elevate PI.
  • Fetal behavioural state matters — active sleep or wakefulness increases cerebral flow and lowers MCA PI. Measure during quiet sleep.

CPR interpretation guide:
  • CPR > 1.0 (or > 5th centile): Normal — no brain-sparing. Blood flow distribution between brain and placenta is balanced.
  • CPR < 1.0 (or < 5th centile): Abnormal — brain-sparing is present. The fetus is redistributing blood toward the brain due to hypoxia. Indicates fetal compromise.
  • CPR < 1.0 with normal UA PI: Typical of late-onset FGR — the placenta is still functioning adequately to maintain UA flow, but the fetus is already experiencing subtle hypoxia and is brain-sparing.
  • CPR < 1.0 with elevated UA PI: More advanced placental insufficiency — both brain-sparing and elevated placental resistance are present.
  • CPR normalising after being low: May indicate loss of brain-sparing (decompensation) — the brain can no longer maintain vasodilation. This is a dangerous sign suggesting impending fetal demise.

CPR vs. individual UA and MCA Doppler:
  • UA PI alone: Tells you about placental resistance, but cannot detect cerebral redistribution. May be normal in late-onset FGR.
  • MCA PI alone: Tells you about cerebral resistance, but cannot tell you if the cause is hypoxia or a normal variant.
  • CPR (MCA PI ÷ UA PI): Combines both — detects the redistribution between brain and placenta. More sensitive than either alone for detecting early fetal compromise.
  • CPR is complementary — not a replacement for UA and MCA PI individually. Report all three.

Normal CPR Reference Values by Gestational Age

GA (weeks)	CPR (5th–95th centile)	Mean CPR	Fixed cut-off	Interpretation
20	1.40–2.50	1.95	< 1.0	High cerebral resistance; low placental resistance — normal
22	1.35–2.40	1.88	< 1.0	—
24	1.30–2.30	1.80	< 1.0	—
26	1.25–2.20	1.73	< 1.0	—
28	1.20–2.10	1.65	< 1.0	—
30	1.15–2.00	1.58	< 1.0	—
32	1.10–1.90	1.50	< 1.0	CPR beginning to decline — monitor in late FGR
34	1.05–1.80	1.43	< 1.0	—
36	1.00–1.70	1.35	< 1.0	Lower end of normal approaches 1.0 — use centiles, not fixed cut-off
38	0.95–1.60	1.28	< 1.0	5th centile may dip below 1.0 — centile-based assessment essential
40	0.90–1.50	1.20	< 1.0	At term, CPR is naturally lower — always use GA-specific centiles
41–42	0.85–1.45	1.15	< 1.0	Post-term CPR may fall further — interpret with caution

Clinical Decision Thresholds (CPR)

CPR Finding	Pathophysiology	UA PI	MCA PI	Clinical Action
CPR > 5th centile (normal)	No brain-sparing; normal flow distribution	Normal	Normal	Routine surveillance
CPR < 5th centile (abnormal) with normal UA PI	Early brain-sparing; typical of late-onset FGR	Normal	Low or normal-low	Serial Doppler; weekly monitoring; assess growth; consider delivery at 37–38 weeks
CPR < 5th centile with elevated UA PI	Established placental insufficiency + brain-sparing	Elevated	Low	Close monitoring; consider delivery from 34–37 weeks depending on severity
CPR < 1.0 (fixed cut-off)	Brain-sparing — MCA PI < UA PI	Variable	Low	Assess overall clinical picture; serial Doppler; plan delivery timing
CPR normalising after being low (MCA PI rising back)	Loss of brain-sparing — decompensation	Elevated	Rising / normalising	Danger sign — deliver immediately if viable
CPR < 0.6765 MoM	Research / centile-based abnormal	Variable	Variable	Interpret as per local protocol

CPR Findings in Different Clinical Scenarios

Scenario	UA PI	MCA PI	CPR	Interpretation
Normal fetus	Normal (low)	Normal (higher than UA)	> 1.0 (normal)	Healthy placenta; no brain-sparing
Early brain-sparing (hypoxia beginning)	Normal	Falling	Falling, may cross < 5th centile	Earliest sign of fetal compromise; UA still normal
Late-onset FGR (≥32 weeks)	Normal	Low	< 5th centile	Typical pattern — CPR is the only abnormal Doppler finding
Early-onset FGR with placental insufficiency	Elevated	Low	< 1.0 (very low)	Both placental resistance and brain-sparing are present
Advanced FGR — decompensation	Very high (AEDF/REDF)	Normalising / rising	May appear to "improve"	Loss of brain-sparing — brain can no longer compensate; impending demise
Post-term fetus	Normal or slightly high	Normal or slightly low	Borderline low (~1.0)	Interpret with GA-specific centiles — CPR naturally falls at term

CPR vs. other Doppler indices — summary comparison:
  • UA PI: Measures placental resistance. Good for early-onset FGR. Insensitive for late-onset FGR.
  • MCA PI: Measures cerebral resistance. Detects brain-sparing. Less specific when used alone.
  • CPR (MCA PI ÷ UA PI): Combines both. Most sensitive marker for early fetal compromise. Detects brain-sparing before UA or MCA alone are abnormal. Essential for late-onset FGR.
  • CPR is not a standalone test — always interpret with UA PI, MCA PI, and clinical context.

Key points to remember:
  • CPR = MCA PI ÷ UA PI — a composite index comparing cerebral and placental resistance.
  • Normal CPR is > 1.0 (or > 5th centile for GA).
  • CPR < 1.0 (or < 5th centile) indicates brain-sparing — the fetus is shunting blood toward the brain due to hypoxia.
  • CPR is the earliest Doppler marker to become abnormal in FGR — it changes before UA PI, MCA PI alone, or CTG.
  • In late-onset FGR (≥32 weeks), UA Doppler is often normal and CPR may be the only abnormal finding.
  • Always use gestational-age-specific centiles — CPR naturally declines near term, and a fixed cut-off of < 1.0 may miss or over-call cases at term.
  • Loss of brain-sparing (CPR normalising after being low) is a danger sign — indicates fetal decompensation and impending demise.
  • MCA PI must be measured in the proximal third of the vessel, at the level of the thalami, during fetal rest.
  • Do not apply angle correction for PI measurements (both UA and MCA) — PI is angle-independent.
  • Do not press hard on the maternal abdomen — can alter intracranial pressure and falsely elevate MCA PI.
  • CPR is complementary to UA PI and MCA PI — report all three for a complete Doppler assessment.
  • A low CPR is associated with adverse perinatal outcomes and long-term neurodevelopmental impairment, even when UA Doppler is normal.

Cerebroplacental Ratio (CPR) Calculator

Fetal Doppler assessment — Middle Cerebral Artery & Umbilical Artery Pulsatility Index

CPR = MCA-PI ÷ UA-PI
A low ratio indicates "brain-sparing" redistribution, suggesting possible placental insufficiency.

MCA-PI (Middle Cerebral Artery)

UA-PI (Umbilical Artery)

Gestational Age (weeks, optional — for reference context)

Please enter valid positive values for both MCA-PI and UA-PI.

Cerebroplacental Ratio

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Interpretation thresholds used: CPR ≥ 1.0 normal; 0.6765–1.0 borderline / low-normal; < 0.6765 abnormal (often approximated as < 5th centile / low MoM). Always evaluate against gestational-age-specific reference ranges and overall clinical context.

For clinical decision support only. Not a substitute for professional judgment. Always interpret alongside biophysical profile, growth, and full Doppler assessment.

Umbilical Artery (UA) Fetal doppler

UCA Doppler study

Umbilical Artery (UA) Fetal doppler & normal value

Normal Value

Umbilical Artery (UA) Doppler Fetal Sonography Updated 2026 Educational Reference

Umbilical Artery (UA) Doppler

• S/D ratio
• RI (Resistance Index)
• PI (Pulsatility Index)
• EDV (End-Diastolic Velocity)
• PSV (Peak Systolic Velocity)
• End-diastolic flow (present/absent/reversed)
• Diastolic notch

Umbilical Artery (UA) Doppler assesses feto-placental circulation by evaluating blood flow velocity waveforms in the umbilical artery. It reflects downstream placental vascular resistance. As gestation advances, placental vascular bed grows, vascular resistance decreases, and end-diastolic flow increases. Abnormally high resistance indicates placental insufficiency and is associated with intrauterine growth restriction (IUGR), pre-eclampsia, and fetal hypoxia.
S/D ratio → S/D ratio (Systolic/Diastolic ratio) is the ratio of peak systolic velocity (S) to end-diastolic velocity (D) in the umbilical artery waveform. It is one of the simplest and most widely used Doppler indices to estimate downstream placental vascular resistance.
Formula:
  S/D ratio = Peak Systolic Velocity (S) ÷ End-Diastolic Velocity (D)
What it measures:
The umbilical artery carries deoxygenated blood from the fetus to the placenta. The placenta acts as a low-resistance vascular bed. When placental development is normal, blood flows forward throughout the entire cardiac cycle — including diastole — so the D (diastolic) value is high, and the S/D ratio is low. When the placental vascular bed is underdeveloped or damaged (as in pre-eclampsia, IUGR, or placental insufficiency), resistance rises, diastolic flow drops, and the S/D ratio increases.
As gestation advances from 20 to 40 weeks, placental villi proliferate, vascular resistance falls, diastolic flow increases, and the S/D ratio progressively decreases.
• 20 weeks: ~4.0–4.5 (high resistance, diastolic flow just established)
• 28 weeks: ~3.0–3.3 (resistance falling)
• 34 weeks: ~2.5–2.7

Measuring technique for S/D:
1. Identify a free-floating loop of umbilical cord away from placental and fetal insertions.
2. Apply color Doppler to localize the umbilical artery (pulsatile, high-velocity flow).
3. Place PW Doppler sample volume (2–3 mm) over the artery with angle of insonation close to 0°.
4. Measure during fetal rest (no breathing / movement), capture at least 3 uniform waveforms.
5. On each waveform, mark the peak systolic point (S) and the end-diastolic point (D).
6. Most ultrasound machines auto-calculate the S/D ratio; take the mean of 3 waveforms for reporting.
7. Angle correction is not required for ratio-based indices (S/D, RI, PI), as both numerator and denominator are equally affected by the angle.

Normal S/D Ratio Reference Values by Gestational Age

GA (weeks)	S/D Ratio (5th–95th centile)	Interpretation
20	3.5–4.8	High resistance; EDV just appearing
22	3.3–4.5	EDV increasing
24	3.1–4.1	Gradual fall in resistance
26	2.9–3.7	Diastolic notch disappears
28	2.7–3.4	Resistance continuing to fall
30	2.5–3.2	Normal forward diastolic flow
32	2.3–3.0	Steady decline
34	2.2–2.8	—
36	2.0–2.6	—
38	1.9–2.4	—
40	1.8–2.2	Lowest resistance at term
41–42	1.7–2.1	May rise slightly post-term

Abnormal patterns:
• S/D > 95th centile for GA → Elevated placental resistance; early sign of placental insufficiency. Diastolic flow still present but reduced.
• Absent end-diastolic flow (AEDF): D = 0, so S/D ratio becomes undefined (infinite). This is a severe abnormality seen in advanced placental insufficiency.
• Reversed end-diastolic flow (REDF): D is negative; S/D ratio becomes negative. This indicates critical fetal hypoxia / acidemia and is an indication for urgent delivery.

Limitation of S/D ratio:
The S/D ratio is not reliable when end-diastolic flow is absent or reversed, because the ratio becomes infinite or negative. In these situations, the Pulsatility Index (PI) is preferred, as it uses mean velocity and remains mathematically valid even with AEDF or REDF.

Clinical Decision Thresholds (S/D Ratio)

S/D Ratio Finding	Placental Resistance	Clinical Action
Within normal range for GA	Normal	Routine surveillance
> 95th centile (EDV present)	Elevated	Serial Doppler every 1–2 weeks; monitor growth
AEDF (D = 0, ratio = ∞)	Severely elevated	Daily CTG; deliver by 34–37 weeks after steroids
REDF (D negative, ratio < 0)	Critical	Deliver immediately if ≥32 weeks

Key points to remember:
  • S/D ratio decreases with advancing gestation (normal trend).
  • A rising or static S/D ratio across serial scans is concerning even if still within "normal" range.
  • Always interpret S/D ratio against gestational-age-specific centiles, not a single cut-off.
  • S/D ratio is angle-independent but invalid in AEDF/REDF — switch to PI.
  • Measure on a free cord loop, not near insertions, to avoid falsely elevated values.
  • S/D, RI, and PI are correlated; most centres now report PI as the primary parameter, with S/D as supportive.

RI (Resistance Index) →
Resistance Index (RI), also called Pourcelot Index, is a Doppler-derived index that quantifies downstream vascular resistance in the umbilical artery. It compares the difference between peak systolic and end-diastolic velocity with the peak systolic velocity, expressed as a ratio between 0 and 1.

Formula:
  RI = (S − D) ÷ S
  where S = Peak Systolic Velocity, D = End-Diastolic Velocity

What it measures:
The RI reflects the resistance to blood flow in the placental vascular bed downstream of the measurement point.

• When placental resistance is low (normal), diastolic flow (D) is high, so (S − D) is small relative to S, giving a low RI (closer to 0).
• When placental resistance is high (abnormal), diastolic flow (D) drops, so (S − D) approaches S, giving a high RI (closer to 1).
• RI ranges from 0 (no resistance, D = S) to 1 (maximum resistance, D = 0).
As gestation advances, placental villi proliferate and vascular resistance falls, diastolic flow increases, and the RI progressively decreases from approximately 0.75 at 20 weeks to approximately 0.55 at term.
• 20 weeks: RI ~0.75 (high resistance; EDV just established)
• 28 weeks: RI ~0.67 (resistance falling)
• 34 weeks: RI ~0.61
• 40 weeks: RI ~0.55 (lowest resistance at term)

Measuring technique for RI:
1. Identify a free-floating loop of umbilical cord, away from placental and fetal insertion sites.
2. Apply color Doppler to localize the umbilical artery (pulsatile, high-velocity flow, typically red/blue alternating).
3. Place PW Doppler sample volume (2–3 mm) over the artery with the angle of insonation as close to 0° as possible.
4. Ensure the fetus is in a quiet resting state — no fetal breathing, no active movement. If fetal breathing is present, wait for a pause.
5. Capture at least 3–4 consecutive uniform waveforms on the same sweep.
6. On each waveform, the machine auto-detects the peak systolic point (S) and the end-diastolic point (D).
7. The RI is auto-calculated as (S − D) ÷ S for each waveform.
8. Report the mean RI of at least 3 waveforms.
9. Angle correction is NOT required for RI (ratio-based index — angle cancels out).
10. Use the same cord segment for serial follow-up to minimize variability.

Normal RI Reference Values by Gestational Age

GA (weeks)	RI (5th–95th centile)	Mean RI	Interpretation
20	0.58–0.82	0.75	High resistance; EDV just appearing
22	0.56–0.80	0.73	EDV increasing
24	0.55–0.78	0.71	Gradual fall in resistance
26	0.53–0.76	0.69	Diastolic notch disappears
28	0.52–0.74	0.67	Resistance continuing to fall
30	0.50–0.72	0.65	Normal forward diastolic flow
32	0.48–0.70	0.63	Steady decline
34	0.46–0.68	0.61	—
36	0.45–0.66	0.59	—
38	0.43–0.64	0.57	—
40	0.42–0.62	0.55	Lowest resistance at term
41–42	0.41–0.61	0.54	May rise slightly post-term

Abnormal Patern:
  • RI within normal centile for GA: Normal placental resistance.
  • RI > 95th centile: Elevated placental resistance — early placental insufficiency. EDV still present but reduced.
  • RI = 1.0 (AEDF): Absent end-diastolic flow. Severe placental insufficiency. RI has reached its ceiling — cannot grade further severity. Switch to PI.
  • RI > 1.0 (REDF): Reversed end-diastolic flow. RI is now outside its valid range. Critical fetal hypoxia. Delivery indicated.
  • RI < 5th centile: Very low resistance — uncommon; may be seen in conditions like arterio-venous fistula, fetal anemia, or thyrotoxicosis.

RI vs S/D vs PI — comparison:
  • S/D ratio: Simple but becomes infinite in AEDF and negative in REDF.
  • RI: Bounded 0–1, but hits ceiling at AEDF and exceeds range in REDF.
  • PI: Uses mean velocity; remains valid in AEDF and REDF; most robust and preferred index in modern practice.
  • All three are angle-independent and decrease with gestational age.

Advantages of RI:
  • Angle-independent — no angle correction needed, because both S and D are equally affected by the angle of insonation.
  • Simple to calculate — only two measurements (S and D) required.
  • Easy to interpret — bounded between 0 and 1; higher = more resistance.
  • Reproducible across different machines and operators when measured correctly.

Limitations of RI:
  • Invalid when D = 0 (AEDF): RI becomes 1.0 (maximum), but this is a ceiling — it cannot quantify how severe the resistance is beyond that point.
  • Invalid when D is negative (REDF): RI becomes >1, which is outside the normal 0–1 range and clinically misleading.
  • Less robust than PI: RI only considers two points (S and D) on the waveform and ignores the entire waveform shape. PI uses the time-averaged mean velocity, making it more informative, especially in abnormal waveforms.
  • Less sensitive than PI for detecting early or subtle changes in placental resistance.

Clinical Decision Thresholds (RI)

RI Finding	Placental Resistance	Clinical Action
Within normal centile for GA	Normal	Routine surveillance
> 95th centile (EDV present)	Elevated	Serial Doppler every 1–2 weeks; monitor growth
RI = 1.0 (AEDF)	Severely elevated	Daily CTG; deliver by 34–37 weeks after steroids
RI > 1.0 (REDF)	Critical	Deliver immediately if ≥32 weeks
RI < 5th centile	Very low (uncommon)	Investigate fetal anemia, AVM, thyrotoxicosis

Key points to remember:
  • RI decreases with advancing gestation (normal trend).
  • RI is bounded between 0 and 1 in normal and AEDF states; exceeds 1 only in REDF.
  • RI is angle-independent — no angle correction needed.
  • RI hits a ceiling at 1.0 in AEDF and cannot grade severity further — switch to PI.
  • RI is less sensitive than PI for detecting subtle waveform changes.
  • Always interpret RI against gestational-age-specific centiles, not a single cut-off.
  • Measure on a free cord loop, not near insertions, to avoid falsely elevated values.
  • Most modern centres report PI as the primary parameter, with RI as supportive.

PI (Pulsatility Index) →
Pulsatility Index (PI), also known as Gosling Index, is the most robust and most widely recommended Doppler index for assessing umbilical artery blood flow and downstream placental vascular resistance. Unlike S/D ratio and RI, which use only two points on the waveform (peak systole and end-diastole), PI incorporates the entire waveform by using the time-averaged mean velocity, making it the most informative of the three indices.

Formula:
  PI = (S − D) ÷ TAMV
  where S = Peak Systolic Velocity, D = End-Diastolic Velocity, TAMV = Time-Averaged Mean Velocity
TAMV is the mean velocity averaged over one complete cardiac cycle, calculated as the area under the velocity-time curve divided by the cycle duration. It captures the entire shape of the waveform — not just the systolic peak and diastolic trough. This is why PI is more sensitive to subtle changes in waveform morphology than S/D or RI.

What it measures:
PI reflects the pulsatility of blood flow — the degree to which velocity fluctuates between systole and diastole relative to the mean.

• When placental resistance is low (normal), flow is continuous and smooth, the waveform is broad, and the difference between S and D is small relative to the mean → low PI.
• When placental resistance is high (abnormal), diastolic flow drops, the waveform becomes narrow and spiky, and the difference between S and D is large relative to the mean → high PI.
• Unlike RI (bounded 0–1), PI has no fixed upper or lower bound — it can take any positive value, and remains mathematically valid even in extreme flow states.
As gestation advances, placental vascular bed grows, resistance falls, diastolic flow increases, the waveform broadens, and PI progressively decreases from approximately 1.40 at 20 weeks to approximately 0.70 at term.

• 20 weeks: PI ~1.35–1.45 (high resistance; narrow waveform)
• 28 weeks: PI ~1.02–1.15 (resistance falling; waveform broadening)
• 34 weeks: PI ~0.82–0.95
• 40 weeks: PI ~0.65–0.76 (lowest resistance; broad, continuous flow)

Measuring technique for PI:
1. Identify a free-floating loop of umbilical cord, away from placental and fetal insertion sites.
2. Apply color Doppler to localize the umbilical artery (pulsatile, high-velocity flow).
3. Place PW Doppler sample volume (2–3 mm) over the artery with angle of insonation close to 0°.
4. Ensure the fetus is in a quiet resting state — no fetal breathing, no active movement. Wait for a breath-hold pause if needed.
5. Capture at least 3–4 consecutive uniform waveforms on the same sweep.
6. The machine auto-traces the entire waveform envelope — peak systole (S), end-diastole (D), and the time-averaged mean (TAMV) over the cardiac cycle.
7. PI is auto-calculated as (S − D) ÷ TAMV for each waveform.
8. Report the mean PI of at least 3 waveforms.
9. Angle correction is NOT required — PI is a ratio-based index and is angle-independent.
10. Ensure the waveform baseline is clear and there is no aliasing — aliasing distorts the envelope and gives false TAMV and false PI.
11. If end-diastolic flow is absent or reversed, verify manually that the machine has traced the waveform correctly — some older machines may fail to auto-detect D = 0 or D < 0.
12. Use the same cord segment for serial follow-up to minimize variability.

Why PI is the preferred index:
• Uses the entire waveform — captures shape, not just two points. More representative of overall flow dynamics.
• Valid in AEDF (D = 0): PI = (S − 0) ÷ TAMV = S ÷ TAMV. Since TAMV is still measurable (there is still forward systolic flow), PI remains a finite, meaningful number. This is the critical advantage over S/D (which becomes infinite) and RI (which hits ceiling at 1.0).
• Valid in REDF (D < 0): PI = (S − (negative D)) ÷ TAMV = (S + |D|) ÷ TAMV. PI becomes larger but still finite and quantifiable, allowing gradation of severity even in the worst flow states.
• More sensitive to early/subtle changes in placental resistance than S/D or RI.
• Better correlation with adverse perinatal outcomes in research studies.
• Recommended as the primary parameter by most guidelines (ISUOG, ACOG, RCOG) for umbilical artery Doppler assessment.

Normal PI Reference Values by Gestational Age

GA (weeks)	PI (5th–95th centile)	Mean PI	Interpretation
20	1.20–1.55	1.40	High resistance; narrow waveform
22	1.15–1.48	1.34	EDV increasing; waveform broadening
24	1.08–1.40	1.26	Gradual fall in resistance
26	1.00–1.32	1.18	Diastolic notch disappears
28	0.92–1.25	1.08	Resistance continuing to fall
30	0.85–1.18	1.01	Normal forward diastolic flow
32	0.78–1.10	0.95	Steady decline
34	0.72–1.02	0.88	—
36	0.66–0.95	0.82	—
38	0.62–0.88	0.76	—
40	0.58–0.82	0.70	Lowest resistance at term
41–42	0.55–0.80	0.68	May rise slightly post-term

Abnormal Patterns:
  • PI within normal centile for GA: Normal placental resistance and waveform.
  • PI > 95th centile (EDV present): Elevated placental resistance — early placental insufficiency. Waveform shows reduced but present diastolic flow.
  • PI very high + AEDF (D = 0): Absent end-diastolic flow. Severe placental insufficiency. PI remains finite and can grade severity (higher PI = worse). This is PI's key advantage over S/D and RI.
  • PI very high + REDF (D < 0): Reversed end-diastolic flow. Critical fetal hypoxia / acidemia. PI is even higher and still quantifiable — delivery indicated.
  • PI < 5th centile: Very low resistance — uncommon; may indicate high-output states (fetal anemia, AVM, thyrotoxicosis) or sometimes a normally developing placenta with very low resistance.

PI vs S/D vs RI — comprehensive comparison:
  • S/D ratio: Uses S and D only. Becomes infinite in AEDF, negative in REDF. Simple but least robust.
  • RI: Uses S and D only. Bounded 0–1. Hits ceiling at 1.0 in AEDF, exceeds range in REDF. Cannot grade severity beyond AEDF.
  • PI: Uses S, D, and TAMV (entire waveform). No fixed bounds. Remains finite and quantifiable in AEDF and REDF. Most sensitive and most robust. Preferred index.
  • All three are angle-independent and decrease with gestational age.
  • All three require gestational-age-specific centiles for interpretation.

Limitations of PI:
  • Requires accurate mean velocity — depends on the machine correctly tracing the entire waveform envelope. Poor waveform quality or noise can affect TAMV and hence PI.
  • Slightly more operator-dependent than S/D or RI, because the machine must trace the full waveform, not just mark two points.
  • Requires clear, uniform waveforms — arrhythmias, fetal breathing, or movement artifacts can distort the mean and give unreliable PI values.
  • No fixed range (unlike RI's 0–1) — interpretation must always be against gestational-age-specific centiles.
  • Angle-independent (like S/D and RI) — no angle correction needed, since all velocity components are equally affected.

Clinical Decision Thresholds (PI)

PI Finding	Placental Resistance	Waveform	Clinical Action
Within normal centile for GA	Normal	Forward flow; EDV present	Routine surveillance
> 95th centile (EDV present)	Elevated	Reduced EDV; narrow waveform	Serial Doppler every 1–2 weeks; monitor growth
Very high PI + AEDF	Severely elevated	D = 0; no end-diastolic flow	Daily CTG; deliver by 34–37 weeks after steroids
Very high PI + REDF	Critical	D < 0; reversed diastolic flow	Deliver immediately if ≥32 weeks
PI < 5th centile	Very low (uncommon)	Broad, continuous waveform	Investigate fetal anemia, AVM, thyrotoxicosis

Key points to remember:
  • PI decreases with advancing gestation (normal trend).
  • PI uses the entire waveform (S, D, and TAMV) — not just two points.
  • PI is angle-independent — no angle correction needed.
  • PI remains finite and quantifiable in AEDF and REDF — its biggest advantage over S/D and RI.
  • PI is the most sensitive index for detecting early changes in placental resistance.
  • PI is the recommended primary parameter by ISUOG, ACOG, and RCOG.
  • Always interpret PI against gestational-age-specific centiles, not a single cut-off.
  • Measure on a free cord loop, not near insertions, to avoid falsely elevated values.
  • Ensure clear, uniform waveforms with no aliasing — poor waveform quality affects TAMV and hence PI.
  • Report the mean of at least 3 waveforms for reliability.

PSV (Peak Systolic Velocity) → The maximum blood flow velocity measured at the peak of systole in the umbilical artery Doppler waveform. It represents the highest forward velocity achieved during ventricular contraction (systole) and is expressed in centimetres per second (cm/s). Unlike ratio-based indices (S/D, RI, PI), PSV is an absolute velocity measurement, not a ratio — so it is angle-dependent and requires accurate angle correction for reliable quantification.

  PSV = Maximum velocity at the peak of the systolic upstroke in the UA waveform
  Unit: cm/s

What it measures:
PSV reflects the force of the fetal cardiac contraction and the volume of blood ejected into the umbilical artery during systole. It is influenced by:

  • Fetal cardiac output — higher cardiac output → higher PSV
  • Fetal heart rate (FHR) — at higher FHR, systolic ejection time shortens, which can affect PSV
  • Placental vascular resistance — higher downstream resistance → higher systolic peak (blood hits a "stiff" placental bed)
  • Blood viscosity — fetal anemia (low viscosity) → higher PSV; polycythaemia (high viscosity) → lower PSV
  • Umbilical artery diameter — narrower vessel → higher velocity (continuity equation: A₁V₁ = A₂V₂)

PSV increases with advancing gestational age. This is because the fetal cardiac output rises as the fetus grows, the umbilical artery diameter increases, and the overall blood flow volume increases. Despite the fall in placental resistance (which would tend to lower systolic peak), the dominant effect of increasing cardiac output causes PSV to rise from approximately 35–45 cm/s at 20 weeks to approximately 55–70 cm/s at term.

  • 20 weeks: PSV ~35–45 cm/s
  • 28 weeks: PSV ~45–58 cm/s
  • 34 weeks: PSV ~52–64 cm/s
  • 40 weeks: PSV ~56–70 cm/s

Measuring technique for PSV:
  1. Identify a free-floating loop of umbilical cord, away from placental and fetal insertion sites.
  2. Apply color Doppler to visualize the umbilical artery — note the direction of flow (colour: red = toward probe, blue = away).
  3. Place PW Doppler sample volume (2–3 mm) over the artery.
  4. Adjust the angle of insonation to ≤ 30° by manoeuvring the transducer or using beam steering.
  5. Activate angle correction on the machine — align the correction line parallel to the vessel wall / direction of flow shown on color Doppler.
  6. Ensure the fetus is in a quiet resting state — no breathing, no movement. Wait for a breath-hold pause if needed.
  7. Capture at least 3–4 consecutive uniform waveforms.
  8. The machine auto-detects the peak of the systolic upstroke — this is the PSV.
  9. Report the mean PSV of at least 3 waveforms.
  10. Always apply angle correction — unlike S/D, RI, PI, PSV is NOT angle-independent.
  11. Avoid aliasing — if the systolic peak is cut off (wrapped around), increase the scale / PRF to capture the full peak. Aliasing underestimates PSV.
  12. Use the same cord segment and angle for serial follow-up to minimize variability.

Normal PSV Reference Values by Gestational Age

GA (weeks)	PSV (5th–95th centile) cm/s	Mean PSV cm/s	Interpretation
20	30–50	40	Lower velocities; small vessel, low cardiac output
22	33–53	43	Gradual increase
24	36–56	46	—
26	39–59	49	—
28	42–62	52	Cardiac output rising
30	45–64	54	—
32	47–66	56	—
34	49–68	58	—
36	51–70	60	—
38	52–71	61	Approaching peak
40	53–72	62	Highest velocities at term
41–42	52–70	61	May plateau or slightly decline

Why PSV is different from S/D, RI, PI:
  • S/D, RI, and PI are ratio-based indices — they compare systolic and diastolic velocities relative to each other. They are angle-independent.
  • PSV is an absolute velocity — it tells you how fast the blood is moving at systolic peak, not just the shape of the waveform.
  • PSV is angle-dependent — the measured velocity is affected by the angle between the ultrasound beam and the direction of blood flow.
  • PSV provides complementary information to the ratio indices — it tells you about cardiac output and flow volume, while S/D, RI, PI tell you about downstream resistance.

Angle dependence — the key issue:
The Doppler equation is: V = (c × f_d) ÷ (2 × f₀ × cos θ)
where θ is the angle of insonation. The measured velocity depends on cos θ:

  • θ = 0° → cos θ = 1.0 → measured velocity = true velocity (ideal)
  • θ = 30° → cos θ = 0.87 → measured velocity = 87% of true velocity (13% underestimation)
  • θ = 60° → cos θ = 0.50 → measured velocity = 50% of true velocity (50% underestimation)
  • θ = 90° → cos θ = 0 → no Doppler shift, no velocity measured (perpendicular = invisible)

Therefore, for PSV to be accurate:
  • Keep the angle of insonation ≤ 30° (ideally as close to 0° as possible).
  • Apply angle correction on the machine — align the angle correction cursor with the direction of blood flow shown on color Doppler.
  • Without angle correction, PSV is unreliable and can only be used as an estimate.

Clinical significance of PSV:
  • Normal PSV for GA: Normal fetal cardiac output and placental flow.
  • Low PSV: May indicate reduced fetal cardiac output, fetal hypoxia, myocardial depression, or low blood volume. Can also be falsely low due to poor angle correction or aliasing.
  • High PSV: May indicate increased placental resistance (the fetal heart pumps harder against a stiff placental bed), high cardiac output states (fetal anemia, thyrotoxicosis), or narrowed vessel diameter.
  • PSV is less commonly used alone in UA Doppler — it is usually reported alongside PI to provide both flow-volume and resistance information.
  • PSV is more important in other vessels — e.g., MCA PSV for fetal anemia detection (where PSV > 1.5 MoM is a key criterion for middle cerebral artery).

PSV in the context of other UA Doppler parameters:
  • PSV + PI together: PSV tells you about cardiac output / flow volume; PI tells you about downstream resistance. Together they give a complete picture.
  • PSV rising + PI rising: Increasing cardiac output against rising resistance — compensation phase.
  • PSV falling + PI rising: Failing cardiac output against high resistance — decompensation. Fetal deterioration.
  • PSV rising + PI falling: Normal gestational trend — increasing cardiac output with falling resistance. Healthy.

Factors Affecting PSV Measurement

Factor	Effect on PSV	Notes
Angle of insonation > 30°	Underestimates PSV	Always angle-correct; keep ≤ 30°
Aliasing	Underestimates PSV (peak wrapped)	Increase scale / PRF to unwrap
Fetal breathing / movement	Distorts waveform	Measure during fetal rest
Fetal tachycardia	May alter systolic ejection time	Average multiple waveforms
Fetal anemia	Increases PSV (low viscosity, high output)	Consider MCA PSV for anemia screening
Placental insufficiency	May increase PSV (pump against high resistance)	Interpret with PI
Measurement near insertion sites	Falsely elevated PSV	Use free-floating cord loop
High wall filter	May remove low-velocity diastolic component	Use low wall filter setting

Clinical Decision Thresholds (PSV)

PSV Finding	Possible Cause	Clinical Action
Within normal centile for GA	Normal cardiac output and placental flow	Routine surveillance
Low PSV (< 5th centile)	Reduced cardiac output, fetal hypoxia, myocardial depression	Correlate with PI, CTG, biophysical profile
High PSV (> 95th centile)	High resistance, high-output state, anemia, narrow vessel	Correlate with PI and MCA PSV; investigate
PSV falling on serial scans	Fetal decompensation — failing cardiac output	Urgent fetal assessment; consider delivery
PSV rising with PI rising	Compensation — heart pumping harder against resistance	Close monitoring; serial Doppler
PSV rising with PI falling	Normal gestational trend	Reassuring

Key points to remember:
  • PSV is an absolute velocity (cm/s), not a ratio — it is angle-dependent.
  • Always apply angle correction and keep the angle ≤ 30°.
  • PSV increases with advancing gestation (opposite to S/D, RI, PI which decrease).
  • PSV reflects fetal cardiac output and flow volume, while S/D, RI, PI reflect downstream resistance.
  • PSV is complementary to PI — report both for a complete assessment.
  • Aliasing underestimates PSV — always check the scale and increase PRF if the peak is cut off.
  • PSV is less useful alone in UA Doppler than in MCA Doppler (where MCA PSV is the primary tool for fetal anemia).
  • Measure on a free cord loop, not near insertions, to avoid falsely elevated values.
  • Report the mean of at least 3 waveforms for reliability.
  • Always interpret against gestational-age-specific centiles, not a single cut-off.

EDV (End-Diastolic Velocity) → The lowest blood flow velocity measured at the end of diastole in the umbilical artery Doppler waveform — just before the next systolic upstroke   EDV = Minimum forward velocity at end-diastole (point D on the waveform)
  Unit: cm/s

What it measures:
EDV is the single most important qualitative marker of placental vascular resistance. While indices like S/D, RI, and PI are numbers you compare to centiles, EDV is primarily assessed by its presence, absence, or reversal:
  • EDV present (forward): Placental resistance is low enough to allow continuous forward flow even during diastole. Normal.
  • EDV absent (AEDF): Placental resistance is so high that no blood flows forward during diastole. Severe abnormality.
  • EDV reversed (REDF): Resistance is critical — blood actually flows backward during diastole. Worst prognosis.

As gestation advances, placental resistance falls, and EDV increases throughout gestation. In early pregnancy (before ~16–18 weeks), end-diastolic flow may be absent normally because placental resistance is high. By 20 weeks, EDV should appear and progressively increase. The absence of EDV after 20 weeks is abnormal.

  • 20 weeks: EDV ~8–12 cm/s (just appearing, low)
  • 28 weeks: EDV ~14–19 cm/s (clearly present)
  • 34 weeks: EDV ~19–24 cm/s
  • 40 weeks: EDV ~25–30 cm/s (highest at term) increases with gestational age. This is because:
  • Placental resistance falls → less opposition to diastolic flow → EDV rises.
  • Fetal cardiac output increases → more blood volume entering the umbilical circulation → higher systolic and diastolic flow.
  • Umbilical artery diameter increases → lower resistance to flow → higher EDV.

Measuring technique for EDV:
  1. Identify a free-floating loop of umbilical cord, away from placental and fetal insertion sites.
  2. Apply color Doppler to localize the umbilical artery.
  1. Place PW Doppler sample volume (2–3 mm) over the artery with angle of insonation ≤ 30°.
  4. Ensure the fetus is in a quiet resting state — no breathing, no movement.
  5. Capture at least 3–4 consecutive uniform waveforms.
  6. The machine auto-detects the lowest point of the waveform just before the next systolic upstroke — this is EDV.
  7. Report the mean EDV of at least 3 waveforms.
& UA Doppler — the most important qualitative finding is the presence, absence, or reversal of EDV.
    • All ratios (S/D, RI) and PI become invalid or unreliable when EDV = 0 or EDV < 0.
    • Only PI remains mathematically valid in AEDF and REDF.
    • When EDV is absent or reversed, switch to PI for quantification and stop using S/D and RI.

Normal EDV Reference Values by Gestational Age EDV is not routinely reported as a standalone absolute number in most guidelines — its qualitative state (present / absent / reversed) is the primary clinical finding. However, approximate normal ranges are available from reference centile charts.

GA (weeks)	EDV (5th–95th centile) cm/s	Mean EDV cm/s	Interpretation
20	5–15	10	EDV just appearing
22	7–17	11	Gradual increase
24	9–19	13	—
26	10–21	14	—
28	12–22	16	Clearly present
30	13–24	17	—
32	15–25	19	—
34	16–27	21	—
36	18–28	23	—
38	20–30	25	—
40	22–32	27	Highest at term
41–42	21–31	26	May plateau or slightly decline

EDV is angle-dependent: Like PSV, EDV is an absolute velocity, not a ratio. Therefore it is angle-dependent and requires angle correction and an angle of insonation ≤ 30° for accurate quantification. However, in clinical practice, EDV is most often assessed qualitatively — whether it is present, absent, or reversed — rather than by its absolute numeric value, because the qualitative pattern carries the most clinical weight.

Three states of EDV — clinical hierarchy:

  1. EDV present (normal):
    Flow is forward throughout the entire cardiac cycle. The waveform sits entirely above the baseline. This indicates adequate placental vascular development and normal downstream resistance. EDV present but reduced (below 5th centile for GA) suggests rising resistance but is still in the "compensated" phase.

  2. AEDF — Absent End-Diastolic Flow:
    The waveform touches the baseline at end-diastole. No forward flow during diastole. This indicates severe placental insufficiency with loss of >70% of tertiary villi. Associated with high perinatal mortality and morbidity. At this stage:
      • S/D ratio → infinite (undefined)
      • RI → 1.0 (ceiling, cannot grade further)
      • PI → high but finite (only PI can grade severity)
    Management: Daily CTG, hospital admission, corticosteroids, delivery by 34 weeks (or 37 weeks if stable).

  3. REDF — Reversed End-Diastolic Flow:
    The waveform drops below the baseline at end-diastole — blood flows backward during diastole. This is the worst finding and indicates critical fetal hypoxia and impending fetal death. Associated with loss of >85% of tertiary villi. At this stage:
      • S/D ratio → negative (undefined)
      * RI → > 1.0 (outside valid range)
      • PI → very high but finite (only PI remains valid)
    Management: Deliver immediately if fetus is viable (≥32 weeks or per local protocol).

Clinical Decision Thresholds (EDV)

EDV Finding	Placental Resistance	Validity of Indices	Clinical Action
EDV present (forward)	Normal or mildly elevated	S/D, RI, PI all valid	Routine surveillance; monitor growth if EDV reduced
EDV reduced (< 5th centile)	Mildly elevated	S/D, RI, PI all valid	Serial Doppler every 1–2 weeks
AEDF (EDV = 0)	Severely elevated	S/D = ∞; RI = 1.0 (ceiling); only PI valid	Daily CTG; deliver by 34 weeks (or 37 if stable)
REDF (EDV < 0)	Critical	S/D = negative; RI > 1.0; only PI valid	Deliver immediately if ≥32 weeks

Key points to remember:
  • EDV increases with advancing gestation (normal trend).
  • EDV is an absolute velocity (cm/s), so it is angle-dependent — apply angle correction if reporting the numeric value.
  • The most important clinical use of EDV is qualitative: present / absent / reversed.
  • Absent EDV (AEDF) after 20 weeks is abnormal and indicates severe placental insufficiency.
 &lot; • Reversed EDV (REDF) indicates critical fetal hypoxia — delivery is indicated.
  • When EDV = 0 or EDV < 0, S/D and RI become invalid; only PI remains reliable.
  • Always interpret EDV alongside PI and the clinical context.
  • Measure on a free cord loop, not near insertions, to avoid falsely elevated resistance.
  • Report the mean of at least 3 waveforms for reliability.
  • Always interpret against gestational-age-specific centiles, not a single cut-off.

End-Diastolic Flow → Rrefers to the direction and presence of blood flow in the umbilical artery during the diastolic phase of the fetal cardiac cycle — the period of relaxation between two systolic contractions, when ventricular pressure is lowest. It is the single most important qualitative observation in umbilical artery Doppler.

Why diastolic flow matters:
During systole, the fetal heart actively pumps blood, so forward flow is always present (unless there is cardiac failure). During diastole, however, there is no active pumping — blood moves forward only if the downstream placental vascular bed offers low enough resistance to allow continuous forward flow.

  • If the placenta is healthy and well-developed, its huge network of low-resistance capillaries allows blood to keep flowing forward even during diastole → EDF is present.
  • If the placenta is damaged or underdeveloped, resistance is high, and blood cannot flow forward during diastole → EDF is reduced, absent, or reversed.

Therefore, EDF is a direct indicator of placental vascular integrity and feto-placental circulation health.

The three clinical states of EDF:

  1. Present End-Diastolic Flow (Normal):
    The waveform lies entirely above the baseline throughout the entire cardiac cycle. Forward flow is continuous — systolic peak is high, and diastolic velocity never reaches zero.
    • Indicates a well-developed, low-resistance placenta.
    • S/D, RI, and PI are all valid and within normal ranges.
    • EDF should normally be present from ~18–20 weeks onward. Before this, absent EDF can be a normal finding due to high early-pregnancy resistance.
    • Even when present, EDF can be reduced (below 5th centile for GA), which indicates rising resistance — an early warning sign before AEDF develops.

  2. Absent End-Diastolic Flow (AEDF):
    The waveform touches the baseline at end-diastole — flow stops completely during diastole, then resumes with the next systole. There is no forward flow during diastole, but there is no backward flow either.
    • Indicates severe placental insufficiency with destruction or underdevelopment of >70% of tertiary placental villi.
    • Strongly associated with IUGR, pre-eclampsia, fetal hypoxia, oligohydramnios, and perinatal mortality.
    • At this stage, the indices behave as follows:
      – S/D ratio → infinite / undefined (D = 0, division by zero)
      – RI → 1.0 (ceiling — cannot grade severity further)
      – PI → high but finite (the only index that remains quantifiable)
    • Management: Hospital admission, daily CTG monitoring, corticosteroids for fetal lung maturity, delivery by 34 weeks (or 37 weeks if stable and serial Dopplers are static).

  3. Reversed End-Diastolic Flow (REDF):
    The waveform dips below the baseline at end-diastole — blood flows backward during diastole. The placenta offers such extreme resistance that it actually pushes blood back toward the fetus during the relaxation phase.
    • Indicates critical placental failure with loss of >85% of tertiary villi.
    • Strongly associated with fetal acidaemia, impending fetal demise, and very high perinatal mortality.
    • At this stage, the indices behave as follows:
      – S/D ratio → negative (D < 0, so S/D is negative — meaningless)
      – RI → greater than 1.0 (outside valid 0–1 range — meaningless)
      – PI → very high but finite (still quantifiable — only PI works here)
    • Management: Deliver immediately if fetus is viable (≥32 weeks or as per local protocol). Do not wait.

Measuring technique for assessing EDF:
  1. Identify a free-floating loop of umbilical cord, away from placental and fetal insertion sites.
  2. Apply color Doppler to localize the umbilical artery (pulsatile, high-velocity flow).
  3. Place PW Doppler sample volume (2–3 mm) over the artery with angle of insonation as close to 0° as possible.
  4. Ensure the fetus is in a quiet resting state — no breathing, no movement. Wait for a breath-hold pause if needed.
  5. Capture at least 3–4 consecutive uniform waveforms.
  6. Visually inspect the diastolic portion of each waveform — does it stay above, touch, or dip below the baseline?
  7. Confirm EDF state on at least 3 waveforms to avoid misclassification due to artifact or transient variation.
  8. Use a low wall filter — a high wall filter can remove low-velocity diastolic flow and falsely create AEDF.
  9. Use an appropriate scale / PRF — too high a scale can flatten the waveform and make EDF hard to see; too low can cause aliasing.
  10. If EDF appears absent, double-check machine settings (wall filter, scale, gain) before confirming AEDF — a false AEDF from settings error is a serious mistake.
  11. Record and freeze the image showing the EDF pattern for documentation.

  • Before 14–16 weeks: EDF is normally absent — high placental resistance is physiological in early pregnancy.
  • By 18–20 weeks: EDF should appear and become visible on the waveform.
  • From 20 weeks to term: EDF progressively increases as placental resistance falls.
  • After 20 weeks: Absent or reversed EDF is always abnormal.

End-Diastolic Flow States — Summary Table

EDF State	Waveform Appearance	Placental Pathology	S/D Ratio	RI	PI	Perinatal Risk
Present (Normal)	Above baseline throughout	Normal placenta	Valid, normal	Valid, normal	Valid, normal	Low
Reduced	Above baseline but low	Early villous damage	Valid, elevated	Valid, elevated	Valid, elevated	Moderate
Absent (AEDF)	Touches baseline at end-diastole	Severe villous loss (>70%)	Undefined (∞)	1.0 (ceiling)	High but finite	High
Reversed (REDF)	Below baseline at end-diastole	Critical villous loss (>85%)	Negative	> 1.0 (invalid)	Very high, finite	Very high — impending demise

Clinical Management by EDF State

EDF State	Monitoring	Delivery Timing	Setting
Present (Normal)	Routine antenatal care	At term / as obstetrically indicated	Outpatient
Reduced	Serial UA Doppler every 1–2 weeks; growth scan every 2–3 weeks	At term or if PI > 95th centile persistently	Outpatient with close follow-up
AEDF	Daily CTG; twice-weekly Doppler; hospital admission	34 weeks (or 37 if stable and steroids given)	Inpatient
REDF	Continuous CTG; prepare for delivery	Immediately (if ≥32 weeks or per local protocol)	Inpatient — urgent delivery

Abnormal Pattern:
  • High wall filter: Removes low-velocity flow → falsely shows AEDF. Always use the lowest wall filter setting.
  • Inappropriate scale: Too high → diastolic flow flattened and invisible. Too low → aliasing distorts the waveform.
  • Fetal breathing: Creates waveform variation that can mimic transient AEDF or REDF. Wait for apnoea.
  • Fetal movement: Displaces the sample volume out of the vessel, giving an incomplete or noisy waveform.
  • Measurement near insertion sites: Falsely elevated resistance — may show reduced EDF when the free loop is normal.
  • Single waveform assessment: Always confirm on ≥3 waveforms — a single bad waveform can mislead.
  • Poor angle: While EDF presence/absence is angle-independent in principle, a very poor angle can reduce signal-to-noise and make it hard to see low diastolic flow.
The progression of placental disease — the EDF cascade:
As placental insufficiency worsens over time, EDF follows a predictable deteriorating sequence:

    Normal EDF
       ↓
    Reduced EDF (rising PI/RI/S/D, but EDV still above zero)
       ↓
    AEDF (EDV = 0; D touches baseline)
       ↓
    REDF (EDV below baseline; blood flows backward in diastole)

This progression typically unfolds over weeks to days. The interval from AEDF to REDF can be as short as 7–10 days, and REDF may precede fetal death by hours to days. Once REDF is detected, urgent delivery is mandatory.

How EDF interacts with the Doppler indices:
EDF is the foundation on which all ratio-based indices depend. The value of D (end-diastolic velocity) determines whether S/D, RI, and even PI are valid:

  • EDF present (D > 0): All three indices (S/D, RI, PI) are valid. Centile charts can be used.
  • AEDF (D = 0): S/D becomes infinite, RI hits ceiling at 1.0, only PI remains finite and gradable.
  • REDF (D < 0): S/D is negative, RI is >1.0, only PI is finite and gradable.

This is why PI is the recommended primary index — it works in all three states of EDF, while S/D and RI fail when EDF is absent or reversed.


Key points to remember:
  • End-Diastolic Flow is the most important qualitative finding in UA Doppler — more critical than any single number.
  • EDF increases with gestational age; absent EDF is normal before ~16 weeks but always abnormal after 20 weeks.
  • The clinical hierarchy is: Present → Reduced → AEDF → REDF. Each step worsens prognosis.
  • When EDF is absent or reversed, only PI remains valid — S/D and RI fail.
  • AEDF indicates severe placental insufficiency (>70% villous loss); REDF indicates critical failure (>85% villous loss).
  • REDF may precede fetal death by hours to days — urgent delivery is mandatory.
  • Always check wall filter and scale before confirming AEDF — settings errors can create false AEDF.
  • Confirm EDF state on at least 3 consecutive waveforms during fetal rest.
  • Measure on a free-floating cord loop, not near insertion sites.
  • Document the waveform image showing the EDF pattern for the clinical record.
  • EDF progression (normal → AEDF → REDF) typically occurs over days to weeks — serial monitoring allows timely intervention.

Diastolic Notch →
Diastolic Notch is a brief downward deflection (a small "dip") seen on the umbilical artery Doppler waveform during early diastole — immediately following the systolic peak, before the waveform settles into its diastolic trough. It represents a transient moment where flow velocity dips sharply after systole and then recovers before end-diastole.

What it looks like:
  • The waveform rises to a sharp systolic peak (S).
  • Immediately after the peak, instead of smoothly descending toward the diastolic baseline, the waveform dips downward briefly — creating a small "notch" or indentation.
  • After the notch, the waveform rises again slightly and then continues into the diastolic phase, ending at the end-diastolic point (D).
  • The notch gives the waveform a characteristic "peak-and-dip" or "biphasic descent" appearance in early diastole.

What it means:
The diastolic notch is a marker of elevated downstream vascular resistance. Its presence indicates that the placental vascular bed is still offering significant resistance to flow during the transition from systole to diastole.

  • Notch present: Resistance is elevated — the vessel wall and placental bed "recoil" after systole, causing a brief flow reversal or deceleration.
  • Notch absent: Resistance is low enough that flow decelerates smoothly from systole to diastole without any dip — the waveform has a smooth, rounded descent.

Normal physiological trend:
The diastolic notch is normally present in early gestation because placental resistance is high in the first and early second trimester. As the placenta matures and resistance falls, the notch disappears:

  • Before 20 weeks: A diastolic notch is commonly seen and is considered a normal physiological finding.
  • 20–24 weeks: The notch begins to fade as placental resistance falls.
  • By 26–28 weeks: The diastolic notch should disappear completely in a normally developing placenta.
  • After 28 weeks: A persistent diastolic notch is abnormal and indicates elevated placental resistance — an early sign of placental insufficiency, even if S/D, RI, and PI are still within the normal range.

Clinical significance of a persistent notch:
  • A diastolic notch that persists after 26–28 weeks is one of the earliest signs of rising placental resistance — it may appear before the S/D ratio, RI, or PI cross the 95th centile.
  • It is therefore a screening marker for early placental insufficiency and identifies pregnancies at risk of developing pre-eclampsia and IUGR.
  • A persistent notch with normal indices warrants increased surveillance — serial Doppler and growth monitoring.
  • The notch may progress alongside rising indices: notch → elevated PI/RI/S/D → reduced EDV → AEDF → REDF.
  • However, the notch is not always present in placental insufficiency — its absence does not guarantee normal resistance. It is a specific but not sensitive marker.

Diastolic notch vs. other UA Doppler parameters:
  • The notch is a qualitative visual finding — present or absent. It is not a number.
  • It complements the quantitative indices (S/D, RI, PI) and the qualitative EDF assessment.
  • Notch + normal indices: Early warning — resistance is rising but has not yet pushed indices above the 95th centile. Increase surveillance.
  • Notch + elevated indices: Confirmed elevated resistance — placental insufficiency is established.
  • No notch + elevated indices: Elevated resistance without notch — the notch is not always present even when resistance is high. Rely on indices.
  • No notch + normal indices: Normal — low-resistance placenta with smooth waveform.

Important distinction — UA notch vs. uterine artery notch:
  • The uterine artery diastolic notch is a well-established screening marker for pre-eclampsia and is routinely assessed at the 20–24 week anatomy scan. A persistent uterine artery notch after 24 weeks is a strong predictor of pre-eclampsia and IUGR.
  • The umbilical artery diastolic notch is less commonly discussed as a standalone screening tool but follows the same principle — it reflects downstream (placental) resistance.
  • Both notches disappear with gestational age as resistance falls, and both are abnormal if persistent after ~26–28 weeks.
  • In clinical practice, the uterine artery notch is the more widely used screening marker in the second trimester; the UA notch is a supporting observation when assessing UA Doppler waveforms.

Measuring technique for assessing the diastolic notch:
  1. Identify a free-floating loop of umbilical cord, away from placental and fetal insertion sites.
  2. Apply color Doppler to localize the umbilical artery.
  3. Place PW Doppler sample volume (2–3 mm) over the artery with angle of insonation as close to 0° as possible.
  4. Ensure the fetus is in a quiet resting state — no breathing, no movement.
  5. Capture at least 3–4 consecutive uniform waveforms.
  6. Visually inspect the early diastolic portion of each waveform — look for a brief downward deflection or dip immediately after the systolic peak.
  7. Confirm the notch on at least 3 waveforms to ensure it is a genuine finding and not an artifact.
  8. Use an appropriate scale / PRF — too high a scale can flatten the waveform and make a small notch invisible; too low can cause aliasing.
  9. Use a low wall filter — a high wall filter can remove the low-velocity component of the notch and make it disappear falsely.
  10. Record and freeze the image showing the notch (or its absence) for documentation.

Common pitfalls in notch assessment:
  • High wall filter: Removes the low-velocity notch component → falsely shows "no notch." Always use the lowest wall filter.
  • Inappropriate scale: Too high → notch flattened and invisible. Too low → aliasing distorts the waveform.
  • Fetal breathing: Creates waveform irregularity that can mimic or obscure a notch. Wait for apnoea.
  • Fetal movement: Displaces the sample volume and distorts the waveform. Repeat during fetal rest.
  • Measurement near insertion sites: May falsely elevate resistance and create or exaggerate a notch. Use the free loop.
  • Single waveform assessment: A notch may appear on one waveform due to artifact but not on others. Always confirm on ≥3 waveforms.
  • Confusing with aliasing: Aliasing wraps the peak around and can create a false "notch-like" appearance. Check the scale.
  • Overcalling a normal waveform variant: A slight inflection in early diastole is not always a true notch. A true notch is a distinct downward dip below the expected smooth descent line.

Diastolic Notch — Summary Table

Gestational Age	Notch Finding	Interpretation	Action
< 20 weeks	Notch present	Normal (physiological high resistance)	No action — routine care
20–24 weeks	Notch present	May be normal — fading phase	Reassess at 26–28 weeks
24–26 weeks	Notch present	Borderline — should be disappearing	Close follow-up; recheck in 2–4 weeks
> 26–28 weeks	Notch present	Abnormal — elevated placental resistance	Serial UA Doppler; assess for pre-eclampsia / IUGR
> 28 weeks	Notch present + elevated PI/RI/S/D	Confirmed placental insufficiency	Increased surveillance; consider delivery timing
Any GA after 28 weeks	Notch absent	Normal — smooth waveform, low resistance	Routine care (if indices also normal)

Diastolic Notch in the Context of UA Doppler Deterioration

Stage	Waveform Finding	Notch	EDV	Indices (S/D, RI, PI)
1 — Normal	Smooth waveform; broad, continuous flow	Absent	Present, normal	Within normal centiles
2 — Early resistance rise	Notch may appear; waveform slightly narrower	Present (earliest sign)	Present, may be reduced	May still be within normal range or just rising
3 — Elevated resistance	Narrow waveform; reduced diastolic flow	Present or absent	Present, reduced	Above 95th centile
4 — AEDF	Waveform touches baseline at end-diastole	May or may not be present	Absent (D = 0)	S/D = ∞; RI = 1.0; PI high but finite
5 — REDF	Waveform dips below baseline at end-diastole	Not relevant at this stage	Reversed (D < 0)	S/D = negative; RI > 1.0; PI very high, finite

Key points to remember:
  • The diastolic notch is a qualitative visual finding — present or absent — seen in early diastole of the UA waveform.
  • It is normal before 20–24 weeks and should disappear by 26–28 weeks as placental resistance falls.
  • A persistent notch after 28 weeks is abnormal and indicates elevated placental resistance — an early warning sign that may precede a rise in S/D, RI, or PI.
  • The notch is specific but not sensitive — its absence does not guarantee normal resistance. Always interpret alongside PI and EDF.
  • A notch with normal indices warrants increased surveillance — the placenta may be in the early stages of insufficiency.
  • The notch is best assessed on ≥3 consecutive uniform waveforms during fetal rest.
  • Wall filter and scale settings can create or erase a notch — always use a low wall filter and appropriate scale.
  • The uterine artery notch is the more widely used screening marker for pre-eclampsia at 20–24 weeks; the UA notch is a supporting finding when assessing the umbilical artery waveform.
  • The notch may be the first abnormality to appear in the deterioration cascade (normal → notch → elevated indices → reduced EDV → AEDF → REDF).
  • Always document the presence or absence of the notch in the Doppler report.