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.
S/D ratio Calculator
UCA Doppler
What does high S/D ratio mean?
UCA Doppler
Causes of ↑ S/D Ratio
Placental insufficiency
Preeclampsia
Chronic fetal hypoxia
Placental infarction / fibrosis
↑ S/D = ↑ RI = ↑ PI → placental insufficiency
| Parameter | Umbilical Artery (UA) | MCA |
|---|---|---|
| Resistance | ↑ Increased | ↓ Decreased |
| PI | ↑ High | ↓ Low |
| RI | ↑ High | ↓ Low |
| S/D ratio | ↑ High | ↓ Low |
| Diastolic flow | ↓ Reduced / Absent | ↑ Increased |
Clinical Insight
|
Clinical Pattern Normal pregnancy → low resistance → low S/D IUGR → high resistance → high S/D Persistent high S/D → poor prognosis if untreated |
Pathophysiology Placental damage → ↑ vascular resistance ↓ diastolic blood flow ↑ S/D ratio → warning sign of IUGR progression |
Umbilical Artery Doppler:
S/D ratio: 6.6 (Elevated for gestational age)
End diastolic flow: Present
Interpretation:
Umbilical artery shows increased resistance with elevated S/D ratio.
Significance:
Findings are suggestive of increased placental vascular resistance, which may be associated with evolving intrauterine growth restriction (IUGR) and risk of fetal hypoxia.
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.
End of Topic
You have reached the end of this Umbilical Artery S/D Ratio Measurement (Step-by-Step) topic.
This content is intended for educational, training, and clinical reference purposes only.
Declaration:
I, R. K. Mouj, declare that the material presented in this “How to Measure Umbilical Artery S/D Ratio (Step-by-Step)” is intended solely for educational and academic purposes. All descriptions related to fetal Doppler technique, identification of the umbilical artery, appropriate sampling site selection, angle correction, spectral waveform acquisition, calculation of systolic/diastolic (S/D) ratio, and interpretation of normal and abnormal flow patterns are illustrative in nature.
Clinical correlation with gestational age, fetal biometry, placental status, amniotic fluid assessment, and maternal condition is essential. Doppler findings should always be interpreted in conjunction with comprehensive obstetric ultrasound evaluation, institutional protocols, and professional medical judgment before making any diagnostic or management decisions.
