perinatology.com

Umbilical Artery Doppler Reference Ranges -with percentiles BETA TESTING

This calculator provides gestational age–specific reference values for commonly used umbilical artery (UA) impedance indices (S/D ratio, resistance index (RI), and pulsatility index (PI)). You can also enter an observed value to estimate its percentile for gestational age and its MoM (multiple of the median = observed ÷ 50th percentile).

Umbilical artery impedance indices are calculated by measuring Doppler waveforms from the umbilical arteries in a free-floating portion of the cord.

S = systolic peak (maximum velocity); D = end-diastolic flow; 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

Clinical note: In normal pregnancy, UA impedance decreases with advancing gestation as placental villous vascularity increases. Elevated indices (or AEDF/REDF) suggest increased placental resistance (eg, fetal growth restriction).
If diastolic flow is absent/reversed, ratio-based interpretation can be misleading; document AEDF/REDF and follow your local protocol.
How the calculator estimates percentiles:
  • Acharya (2005): The paper reports week-specific percentile values (2.5th–97.5th) for each index. For a gestational age with days (eg, 31+4), the calculator linearly interpolates between adjacent completed weeks on each percentile curve. An observed measurement is assigned a percentile by finding which two percentile curves it falls between (at that gestational age) and interpolating between them.
  • INTERGROWTH-21st (Drukker 2020): The paper provides equations for model parameters (λ, μ, σ). The calculator computes a z score from the observed value using the published formula and converts it to a percentile (Φ(z)×100). Expected 50th and 95th values are computed from the same model (z=0 and z=1.64485). (The z score is displayed only when INTERGROWTH is selected.)

Calculator

Choose reference source, then enter GA as weeks + days.

weeks
days

Index Observed (optional) 50th 95th Estimated percentile MoM
S/D ratio
Resistance index (RI)
Pulsatility index (PI)

Percentiles are calculated without extrapolating beyond the published gestational age window for the selected reference source.

Updated 02/24/2026 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. PMID:15746695
  3. Drukker L, Staines-Urias E, Villar J, et al. International gestational age-specific centiles for umbilical artery Doppler indices: a longitudinal prospective cohort study of the INTERGROWTH-21st Project. Am J Obstet Gynecol. 2020;222(6):602. PMID:31954701  |  Full text (PMC)
Important: All calculations must be confirmed before use. The suggested results are not a substitute for clinical judgment. Neither Perinatology.com 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.