Colour Doppler Calculator — Fetal & Placental Vessels
Umbilical artery, MCA and cerebroplacental ratio — from raw Doppler velocities.
Umbilical Artery Doppler Calculator
Enter the peak systolic velocity (S) and end-diastolic velocity (D) read from the umbilical artery colour Doppler waveform to get S/D ratio, RI and PI. Add mean velocity for a more precise PI, or leave blank for a trapezoidal approximation.
MCA Peak Systolic Velocity Calculator
Middle cerebral artery peak systolic velocity (MCA-PSV) screens for fetal anaemia in alloimmunised or parvovirus-exposed pregnancies. Enter measured PSV and GA to get MoM against the Mari reference curve.
Cerebroplacental Ratio Calculator
Cerebroplacental ratio (CPR) = MCA-PI ÷ UA-PI. Enter both pulsatility indices directly, or use the PI calculated in the UA Doppler and MCA-PSV tabs above.
Doppler indices are one part of overall fetal surveillance — always interpret alongside growth trend, amniotic fluid, biophysical profile, and clinical context.
Need EDD, gestational age or estimated fetal weight too?
Pair this Doppler suite with the OB Wheel and Hadlock EFW calculator for a complete growth-scan workup.
Reference 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.
Colour 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 |
Frequently Asked Questions
What is a colour Doppler test in pregnancy?
A colour Doppler study measures blood flow velocity in fetal and placental vessels — most commonly the umbilical artery, middle cerebral artery (MCA), and sometimes the ductus venosus. It helps assess placental function and detect fetal growth restriction, redistribution of blood flow, and fetal anaemia.
What is a normal umbilical artery S/D ratio?
The UA S/D ratio normally decreases as pregnancy advances, generally falling below 3 after 30 weeks of gestation. A rising S/D ratio above the 95th percentile for gestational age, or absent/reversed end-diastolic flow, suggests increased placental resistance and possible fetal growth restriction.
What does absent or reversed end-diastolic flow mean?
Absent end-diastolic flow (AEDF) means diastolic velocity reaches zero, while reversed end-diastolic flow (REDF) means flow direction reverses during diastole. Both indicate significantly increased placental resistance and are associated with higher stillbirth risk, warranting close surveillance and consideration of early delivery depending on gestational age.
What is MCA-PSV used for in pregnancy?
MCA-PSV is used to non-invasively screen for fetal anaemia, most commonly in pregnancies affected by red cell alloimmunisation or parvovirus B19 infection. A measured MCA-PSV of 1.5 MoM or higher for gestational age is the accepted threshold suggesting moderate-to-severe fetal anaemia.
What is the cerebroplacental ratio (CPR)?
CPR is calculated by dividing MCA-PI by UA-PI. A CPR below approximately 1.0, or below the 5th percentile for gestational age, suggests brain-sparing redistribution of blood flow and is associated with placental insufficiency, even when individual UA and MCA values appear normal.
What is the difference between RI and PI?
Both quantify downstream vascular resistance. RI (Pourcelot index) = (S − D) ÷ S and ranges 0–1. PI = (S − D) ÷ mean velocity and can exceed 1. PI is generally preferred because it stays defined even when diastolic flow is absent or reversed.