Objective: It has been suggested that constriction of the umbilical vein (UV) at the umbilical ring has hemodynamic effects. We aimed to determine the occurrence and extent of such constriction in serial observations.
Methods: This was a prospective longitudinal study of UV velocities at the umbilicus measured at approximately 4-week intervals between 19 and 42 weeks' gestation in 129 low-risk singleton pregnancies. Each participant was examined three to five times. Multilevel modeling was used to construct the reference ranges and to test associations between variables.
Results: Gestational age-specific reference percentiles of UV velocities at the umbilicus were established based on 469 observations. Fetuses were able to alter the UV velocities considerably during the second half of pregnancy, signifying a varying degree of UV constriction. Of a total of 129 fetuses, 56 (43.4%) never had high UV blood velocity (i.e. > 46 cm/s, the highest quartile), 42 (32.6%) fetuses had high UV blood velocity on one occasion and 31 (24.0%) fetuses on two or more occasions. In 36 (27.9%) fetuses the UV velocity at the umbilical ring was > 300% of the mean gestational age-specific reference value at the intra-abdominal section on at least one occasion. Constriction of the UV at the umbilical ring did not affect the pulsatility of the umbilical artery, and was not associated with adverse perinatal outcome in this study.
Conclusions: Low-risk fetuses may well constrict the UV at the abdominal wall with velocities extending over wide ranges on one or more occasions during the second half of pregnancy. Rather than being a risk for complications, the constriction seems to be part of physiological development and possibly a regulatory mechanism.