Umbilical velocimetry in normal pregnancy

Five hundred ninety-six normal pregnancies were evaluated with continuous Doppler to determine the changes in velocimetry between 17 and 42 weeks gestation. The wave-form was identified by its characteristic sharp systolic rise and slow diastolic decay; plotting the S/D ratio as a function of the gestational age reveals a progressive decline in the S/D ratio as gestation progresses. A significant drop occurred around 30 weeks when the mean S/D ratio dropped from 3 (29 weeks) to 2.6 (31 weeks) reflecting decreased placental resistance.