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 ≥5 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 ≥300 %, 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 © 2002 John Wiley & Sons, Ltd.