Yolk sac and umbilicoplacental hemodynamics during early human embryonic development

Objectives To determine the relationship between the yolk sac and umbilicop lacental circulations during the first trimester of pregnancy. Design Sixteen normal singleton pregnancies were included in this longitudi nal study. Transvaginal ultrasonography with color and pulsed Doppler was p erformed at 5+, 7+, 8+ and 10+ gestational weeks. Fetal heart rate and bloo d velocity waveforms of yolk sac, umbilical and chorionic arteries were obt ained. Results The detection rate of arterial blood flow in the yolk sac increased significantly from 5+ (2/16) to 7+ gestational weeks (12/16). Thereafter, it declined significantly and no arterial blood flow was detected at 10+ we eks. When the arterial blood flow was observable in the yolk sac, all wavef orms consisted of continuous diastolic blood flow There were no significant changes in peak systolic velocity (PSV), time-averaged maximum velocity (T AMXV), pulsatility index (PI) or resistance index (RI) values of the yolk s ac arteries during the study period. In umbilical arteries, blood flow was detected in two of 16 patients at 5+ weeks and in all cases thereafter. The mean umbilical artery PSV (SD) increased from 7.2 cm/s (3.7) at 8+ weeks t o 13.0 cm/s (2.9) at 10+ weeks (p = 0.001). No changes in PI values were fo und and the absence of diastolic flow uns typical of the umbilical artery d uring the study period. In chorionic arteries, blood flow was detected in s ix out of 16 subjects at the 5+ week, in 14 out of 16 patients at the 7+ we ek and in every case thereafter. No changes in PSV or TAMXV values were fou nd. Conclusions Out results showed that the arterial signals in the yolk sac ci rculation disappeared and the umbilicoplacental circulation increased betwe en 8+ and 10+ weeks of gestation. This indicates that the placenta replaces the yolk sac as an essential source of blood supply to the embryo at that time.