The umbilical ring--the first rapid in the fetoplacental venous system.

In this study, the effect of the umbilical ring on blood flow velocities in the umbilical vein was examined using Doppler sonography. The maximum blood flow velocity in the umbilical vein was measured just before and behind the umbilical ring in eleven normal singleton pregnancies at 24-34 gestational weeks. The maximum velocity increased in each single case (p < 0.0001), the mean increase was from 16 cm/sec to 31 cm/sec. Thus, the narrowing of the umbilical ring serves as the first rapid in venous fetoplacental circulation. The physiological role of the acceleration of venous blood flow remains to be elucidated. Possible functions include stabilizing venous blood flow or preferential streaming.