Umbilical Artery Doppler Indices for Assessment of Fetal Growth Restriction

Mathematical Definitions

The resistance index (RI) is calculated as (peak systolic velocity − end‑diastolic velocity) ÷ peak systolic velocity, whereas the pulsatility index (PI) uses (peak systolic velocity − end‑diastolic velocity) ÷ time‑averaged mean velocity over the cardiac cycle. 1

Both RI and PI are considered equally acceptable for managing suspected intrauterine growth restriction (IUGR); either the systolic‑to‑diastolic (S/D) ratio or PI can be used for surveillance. 2, 1
When end‑diastolic flow is absent, the S/D ratio cannot be calculated (division by zero), but PI remains calculable and can still be applied to evaluate advanced placental compromise. 1
Neither RI nor PI has demonstrated superiority in sensitivity or specificity for detecting growth restriction; both provide comparable predictive performance. (Evidence level not specified) 2, 1

For optimal reproducibility, RI and PI measurements should be obtained at the abdominal insertion of the umbilical cord, although any cord segment is acceptable if the same site is used consistently. 2, 1
Measurements must be performed when the fetus is not exhibiting breathing movements and the Doppler waveform is uniform to avoid artefactual values. 2, 1
Averaging multiple S/D ratios or PI values is unnecessary in routine clinical practice; a single well‑obtained measurement is sufficient. 2, 1

Both indices reflect downstream placental vascular resistance; higher RI or PI values indicate increased resistance within the fetoplacental circulation. 2