Degree of fetal umbilical venous constriction at the abdominal wall in a low-risk population at 20-40 weeks of gestation

Objectives To determine the degree of constriction of the umbilical vein at the abdominal wall in the second half of pregnancy. Methods A total of 283 low-risk singleton pregnancies were recruited for a cross-sectional study, and examined once at 20-40 weeks of gestation. Two sets of ultrasound measurements of the umbilical vein were taken: one at the fetal end of the umbilical cord and another at the inlet through the abdominal wall, the umbilical ring. The diameter was determined as an average of greater than or equal to5 repeat measurements. The blood velocity was recorded at the same site. Results The time-averaged maximum venous blood velocity in the cord was low (mean 13-19 cm/s during 20-40 weeks of gestation), and the corresponding mean diameter 3.6-8.2 mm. In contrast, the mean of the venous blood velocity at the umbilical ring was 34-41 cm/s and the diameter was 2.8-5.9 mm during the same period. Of 191 pairs of observations, 41 (21%) had a velocity increment of greater than or equal to300 %, which corresponds to a diameter reduction to half or more at the umbilical ring. Conclusion Constriction of the umbilical ring is a common phenomenon in the second half of pregnancy. Copyright (C) 2002 John Wiley Sons, Ltd.