Fetal central blood circulation in the third trimester of normal pregnancy--a longitudinal study. I. Aortic and umbilical blood flow.

Fetal central blood circulation was evaluated in 21 uncomplicated pregnancies every other week from the 27th gestational week till term. Blood flow in the fetal descending thoracic and abdominal aorta and in the intra-abdominal umbilical vein was measured with a combined ultrasound real-time and 2 MHz pulsed Doppler technique. The mean fetal blood velocities were fairly constant at the three measuring sites during the last trimester: 34.6 cm X s-1 (S.D. 5.5), 32.7 cm X s-1 (S.D. 5.5) and 12.6 cm X s-1 (S.D. 3.1), respectively. The aortic diameter increased with gestational age, whereas the umbilical vein diameter increased until the 34th gestational week followed by a stagnation. The mean weight-related blood flow in the fetal thoracic descending aorta decreased slightly towards term (from 240.8 ml X min-1 X kg-1 (S.D. 53.6) in the 28th week to 212.6 ml X min-1 X kg-1 (S.D. 37.3). In the umbilical vein, the corresponding blood flow decrease was linear and more pronounced: from 138.7 ml X min-1 X kg-1 (S.D. 76.0) to 65.2 ml X min-1 X kg-1 (S.D. 14.2). The results indicate that the placental proportion of fetal blood flow decreases with gestational age.