Yolk sac and umbilicoplacental hemodynamics during early human embryonic development

To determine the relationship between the yolk sac and umbilicoplacental circulations during the first trimester of pregnancy.

Sixteen normal singleton pregnancies were included in this longitudinal study. Transvaginal ultrasonography with color and pulsed Doppler was performed at 5+, 7+, 8+ and 10+ gestational weeks. Fetal heart rate and blood velocity waveforms of yolk sac, umbilical and chorionic arteries were obtained.

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+ weeks. When the arterial blood flow was observable in the yolk sac, all waveforms consisted of continuous diastolic blood flow. There were no significant changes in peak systolic velocity (PSV), time-averaged maximum velocity (TAMXV), pulsatility index (PI) or resistance index (RI) values of the yolk sac 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 to 13.0 cm/s (2.9) at 10+ weeks (p = 0.001). No changes in PI values were found and the absence of diastolic flow was typical of the umbilical artery during the study period. In chorionic arteries, blood flow was detected in six out of 16 subjects at the 5+ week, in 14 out of 16 patients at the 7+ week and in every case thereafter. No changes in PSV or TAMXV values were found.

Our results showed that the arterial signals in the yolk sac circulation disappeared and the umbilicoplacental circulation increased between 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.