1Reference Doppler Values by Gestational Age
Approximate 95th-percentile upper limits for umbilical artery indices, and median MCA-PSV, by gestational week. Use alongside the calculator above — values trending toward or beyond the 95th percentile warrant closer surveillance.
| GA (weeks) | UA S/D (95th %ile, approx.) | UA PI (95th %ile, approx.) | MCA-PSV Median (cm/s) |
|---|---|---|---|
| 24w | 4.0 | 1.50 | 26.9 |
| 26w | 3.7 | 1.40 | 28.2 |
| 28w | 3.4 | 1.32 | 29.6 |
| 30w | 3.2 | 1.24 | 31.0 |
| 32w | 3.0 | 1.17 | 32.5 |
| 34w | 2.8 | 1.10 | 34.1 |
| 36w | 2.6 | 1.04 | 35.7 |
| 38w | 2.4 | 0.98 | 37.5 |
| 40w | 2.2 | 0.93 | 39.2 |
Values are indicative averages compiled from commonly cited obstetric Doppler reference curves (including Mari et al. for MCA-PSV); confirm against your unit's validated local reference charts for individual patient management.
2Colour Doppler Reading — Clinical Guide
What Does a Colour Doppler Reading Show?
An obstetric colour Doppler study visualises and measures blood flow velocity within fetal and placental vessels — most commonly the umbilical artery (UA), middle cerebral artery (MCA), and occasionally the ductus venosus and uterine arteries. It is used to assess placental function, detect fetal growth restriction, identify redistribution of blood flow ("brain-sparing"), and screen for fetal anaemia.
Umbilical Artery (UA) Doppler — S/D Ratio, RI, PI
The UA waveform reflects placental vascular resistance. As placental resistance falls with advancing gestation, the S/D ratio normally decreases, typically below 3 after 30 weeks. Resistance Index (RI) = (S − D) ÷ S, ranging 0–1. Pulsatility Index (PI) = (S − D) ÷ mean velocity, and is generally preferred clinically because it remains calculable even when end-diastolic flow is absent or reversed — findings that indicate significantly elevated placental resistance and warrant urgent review.
MCA-PSV — Screening for Fetal Anaemia
Middle cerebral artery peak systolic velocity rises when a fetus is anaemic, as reduced blood viscosity and a hyperdynamic circulation increase cerebral flow velocity. Using the Mari reference curve, an MCA-PSV of 1.5 multiples of the median (MoM) or higher for gestational age is the accepted threshold suggesting moderate-to-severe fetal anaemia — most relevant in red cell alloimmunisation (Rh disease), parvovirus B19 infection, or unexplained hydrops.
Cerebroplacental Ratio (CPR)
CPR = MCA-PI ÷ UA-PI. Even when UA and MCA values individually look within normal limits, a falling CPR — generally below about 1.0, or below the 5th percentile for gestational age — reflects early brain-sparing redistribution and is an independent marker of placental insufficiency and adverse perinatal outcome, particularly useful near term.
RI vs PI vs S/D Ratio — Comparison
| Index | Formula | Range | Notes |
|---|---|---|---|
| S/D Ratio | S ÷ D | 1 → ∞ | Undefined if D = 0 (AEDF) |
| Resistance Index (RI) | (S − D) ÷ S | 0 – 1 | Also called Pourcelot index |
| Pulsatility Index (PI) | (S − D) ÷ Mean | 0 → ∞ | Preferred — stays defined with absent/reversed flow |