The development of high venous velocity at the fetal umbilical ring duringgestational weeks 11-19

Objective To determine the occurrence of high venous velocities at the umbi lical ring in the normal early second trimester, based on the assumption th at a narrow umbilical ring may cause obstruction and increased venous blood velocity tit the abdominal wall. Design Cross-sectional study. Setting Hospital antenatal clinic. Population One hundred and one low risk singleton pregnancies specifically recruited for the study. Methods Ultrasound was used at 11-19 weeks to determine the diameter and ve locity in the umbilical vein at the fetal end of the cord and at the inlet through the abdominal wall. Outcome measures 10th, 50th and 90th centiles w ere estimated for the time-averaged maximum velocity in the cord and at the abdominal inlet. The increase of velocity as the blood entered the, abdomi nal wall was calculated in percent of the velocity in the cord. Results During weeks 11-12 there was hardly any difference between blood ve locity in the umbilical vein at the umbilical ring and that in the cord. Fr om week 13 onwards it was increasingly common to find blood acceleration at the umbilical ring of 50-500%. Velocity increment > 50% was found in 0/12 fetuses (0%) at 11-12 weeks, 5/20 (25%) at 13-13 weeks, and in 21/28 (75%) at 17-19 weeks. Conclusions Blood velocity is higher in the umbilical vein at the abdominal wall than the cord, particularly after 13 weeks of gestation. If accelerat ion of blood velocity at the umbilical ring is a sign of a narrow inlet, it seems that a progressive tightening occurs during the second trimester.