When should the umbilical artery systolic to diastolic ratio be measured?
In any setting of oligohydramnios with intact membranes or in any setting of IUGR, Doppler US imaging of the umbilical artery should be performed as a means to assess for placental insufficiency. Normally, the S/D ratio in the umbilical artery gradually decreases during pregnancy as diastolic flow increases with increasing gestational age. Elevated S/D ratios (>4.0 at 26 to 30 weeks, >3.5 at 30 to 34 weeks, and >3 after 34 weeks) indicate diminished diastolic flow, a sign of increased vascular resistance in the placenta, and are associated with increased risk of perinatal morbidity and mortality. Absent or reversed end diastolic flow in the umbilical artery indicates a particularly poor prognosis and usually prompts immediate delivery.
