Umbilical artery velocity waveforms in early pregnancy: a transvaginal color Doppler study.

In a cross-sectional study, umbilical artery velocity waveforms were recorded in 214 low-risk pregnancies at 7 weeks to 16 weeks, menstrual age, by means of transvaginal color and pulsed Doppler ultrasonography. In all the cases studied, end diastolic velocities were absent until the 10th week. From this age onward end diastolic velocities were present in a percentage of pregnancies, progressively increasing with gestation and reaching 100% at 15 weeks. Similarly, the percentage of cardiac cycles in which end diastolic velocities were absent progressively decreased with advancing menstrual age. The normal range for the pulsatility index was constructed and a quadratic function was found to optimally fit its fall during gestation. No differences in pulsatility index values were found at these menstrual ages in 12 pregnancies that later developed intrauterine growth retardation and/or pregnancy-induced hypertension, suggesting that placental alterations causing abnormalities in umbilical velocity waveforms occur later in gestation.