Umbilical Artery S/D Ratio
Fetal Doppler
Introduction
Umbilical artery Doppler is a key tool in fetal surveillance, especially in cases of suspected placental insufficiency and intrauterine growth restriction (IUGR). The S/D ratio reflects downstream placental resistance.
Patient Position
- Supine or slight left lateral position
- Use 3–5 MHz curvilinear probe
Sampling Technique
- Select a free-floating loop of umbilical cord
Free-floating loop [UC]
- Avoid fetal abdominal and placental insertion sites
- Use color Doppler for vessel identification
- Apply pulsed Doppler with angle ≤ 30°
- Sample gate: 1–2 mm
Waveform Criteria
- Obtain at least 3–5 uniform waveforms
- No fetal movement or breathing artifacts
- Clear systolic and diastolic components
Measurement
S/D ratio is calculated as:
S/D = Peak Systolic Velocity ÷ End Diastolic Velocity
Example
S = 40 cm/s, D = 10 cm/s → S/D = 4.0
Normal Values (Gestational Age)
| Gestational Age (Weeks) | Normal S/D Ratio |
|---|---|
| 20 | < 5.0 |
| 24 | < 4.5 |
| 28 | < 4.0 |
| 30 | < 3.8 |
| 32 | < 3.6 |
| 34 | < 3.4 |
| 36 | < 3.2 |
| 38 | < 3.0 |
| 40 | < 3.0 |
Note: S/D ratio decreases with advancing gestational age due to reduced placental resistance.
What does high S/D ratio mean?
UCA Doppler
What does high S/D ratio mean?
UCA Doppler
Abnormal Findings
- Increased S/D ratio → placental resistance
- Absent end-diastolic flow (AEDF)
- Reversed end-diastolic flow (REDF)
Common Errors
- Wrong sampling site
- High Doppler angle
- Large sample volume
- Irregular waveform
Conclusion
Accurate S/D measurement requires proper technique, correct sampling site, and consistent waveform acquisition. It plays a vital role in fetal monitoring and clinical decision-making.
Declaration
This content is intended for academic and educational purposes only.
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