Normal Umbilical Artery S/D Ratio at 38 Weeks
The normal umbilical artery systolic-to-diastolic (S/D) ratio at 38 weeks gestation is approximately 2.1–2.2, with values above the 95th percentile for gestational age (typically >3.0–3.3 at this stage) considered abnormal. 1, 2
Specific Normal Values at 38 Weeks
In normally growing fetuses, the umbilical artery S/D ratio demonstrates a physiologic decline throughout pregnancy, reaching 2.18 ± 0.22 at 38 weeks in one well-characterized cohort of diabetic pregnancies with normal placental function 1
The 95th percentile upper limit of normal at 38 weeks is approximately 3.09–3.35, depending on whether single or averaged measurements are used 2
Normal pregnancies show a progressive decline in S/D ratio from approximately 3.9–4.2 at 20 weeks to 2.1 at 40 weeks, reflecting decreasing placental vascular resistance as tertiary villi mature 1, 3
Clinical Interpretation Framework
Abnormal values are defined as:
- S/D ratio >95th percentile for gestational age (generally >3.0–3.3 at term) 4
- Absent end-diastolic velocity (AEDV) 4
- Reversed end-diastolic velocity (REDV) 4
When S/D ratios are elevated but still show forward diastolic flow:
- Values persistently ≥3.0 in the third trimester warrant increased surveillance for fetal growth restriction and placental insufficiency 1, 3
- A marked increase of ≥20% above the patient's own baseline (even if within population "normal" range) carries increased risk of fetal compromise, with sensitivity 43% and specificity 90% for adverse outcomes 5
Surveillance Implications Based on S/D Values
For normal S/D ratios (<95th percentile) with fetal growth restriction:
- Serial umbilical artery Doppler every 2–4 weeks if stable 4
- Delivery recommended at 38–39 weeks when estimated fetal weight is 3rd–10th percentile 6, 7
For elevated S/D ratios (>95th percentile) without AEDV/REDV:
Important Clinical Caveats
The S/D ratio does not require correction for fetal heart rate in the clinically relevant range of 120–160 bpm, as the correlation is statistically significant but clinically negligible (mean difference only 4.4%) 2
Waveforms can be obtained from any segment of the umbilical cord; placental-end measurements show slightly higher end-diastolic flow, but this variation does not affect clinical decision-making 4
Pulsatility index (PI) and resistance index (RI) are now more commonly used than S/D ratio in contemporary practice, though all three indices correlate well and abnormality is defined similarly (>95th percentile) 4