Fetal umbilical venous flow measured in utero by pulsed Doppler and B-mode ultrasound. I. Normal pregnancies.

Umbilical venous blood flow was measured with a pulsed Doppler unit, which was used in combination with a B-mode ultrasonic imaging system to permit location of the umbilical vein and measurement of its cross-sectional area. The accuracy and limitations of the method are discussed. Forty-seven normal fetuses with gestational ages ranging from 22 weeks to term were studied on a total of 61 occasions. Flow increased with gestational age until 36 weeks, was maximal between 37 and 38 weeks, then decreased during the last 2 weeks of pregnancy. Flow per unit of fetal weight was constant during pregnancy until 36 to 37 weeks, when a reduction occurred.