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Umbilical Artery Doppler Reference Ranges

The calculator estimates the 50th and 95th percentile values for commonly used umbilical artery impedance indices (S/D ratio, resistance index, and pulsatility index) based on gestational age.

Umbilical artery (UA) impedance indices are calculated by using ultrasound to measure the blood flow waveforms from the umbilical arteries through a free-floating portion of the umbilical cord.

S = systolic peak (maximum velocity); the maximum velocity during contraction of the fetal heart.

D = end-diastolic flow; continuing forward flow in the umbilical artery during the relaxation phase of the heartbeat.

Vm = mean velocity.

The ratios (indices) of resistance to blood flow (impedance) are calculated as:

S/D ratio = systolic velocity ÷ diastolic velocity

Resistance index (RI) = (systolic − diastolic) ÷ systolic

Pulsatility index (PI) = (systolic − diastolic) ÷ mean velocity

In the normal fetus, resistance to flow (impedance) decreases in the umbilical artery due to increasing numbers of tertiary stem villi as the placenta matures.

Clinical note: Umbilical artery impedance indices increase when there is decreased end-diastolic flow due to reduced placental perfusion and utero-placental insufficiency, as is seen in intrauterine growth restriction (IUGR). Absent or reversed UA end-diastolic flow are particularly ominous findings.

Calculator

Reference ranges established for approximately 19–41 weeks at a free-loop segment of the umbilical cord [2].
weeks

 

Index 50th percentile 95th percentile
S/D ratio
Resistance index (RI)
Pulsatility index (PI)
Updated 11/17/2025 Mark Curran , MD FACOG

References

  1. Gosling RG, et al. The quantitative analysis of occlusive peripheral arterial disease by non-intrusive ultrasound technique. Angiology. 1971;22:52–5. PMID:5101050
  2. Acharya G, Wilsgaard T, Berntsen GKR, Maltau JM, Kiserud T. Reference ranges for serial measurements of umbilical artery Doppler indices in the second half of pregnancy. Am J Obstet Gynecol. 2005;192:937–44. PubMed
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