S/D Ratio: Doppler ratio of peak systolic to end-diastolic velocity; used primarily in umbilical artery surveillance for IUGR and placental insufficiency.
The systolic-to-diastolic (S/D) ratio is a spectral Doppler measurement comparing peak systolic to end-diastolic velocity in a vessel. It is primarily used in obstetric Doppler assessment of the umbilical artery — a normal S/D ratio decreases with advancing gestation as placental resistance falls. Absent end-diastolic flow (infinite S/D ratio) or reversed EDV indicates critical placental insufficiency.