Umbilical Artery Doppler Reference Ranges

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Umbilical Artery (UA) Impedance Indices are  calculated by using ultrasound to  measure the blood flow waveforms from the uterine arteries through a free-floating portion of the umbilical cord .

The figure at the right shows the normal saw-tooth appearance of arterial blood flow through the umbilical cord.

S = Systolic peak (max 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 the resistance to blood flow (impedance) are calculated using the following formulas:

S/D ratio = (systolic / diastolic ratio)

Resistance index (RI) = (systolic velocity - diastolic velocity / systolic velocity)

Pulsatility index (PI) = (systolic velocity - diastolic velocity / mean velocity)

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

The calculator below will estimate the 50th and 95th percentile values for some  commonly used umbilical artery impedance indices.

 Gestational Age in Weeks   

50th percentile 95th percentile
S/D ratio =
Resistance index =
Pulsatility index =

The umbilical artery (UA) 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.


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 et al. 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

All calculations must be confirmed before use. The suggested results are not a substitute for clinical judgment. Neither nor any other party involved in the preparation or publication of this site shall be liable for any special, consequential, or exemplary damages resulting in whole or part from any user's use of or reliance upon this material.

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