Sexual Dimorphism in Umbilical Vein Blood Flow During the Second Half of Pregnancy: A Longitudinal Study.

Objectives: To investigate gestational age-specific serial changes in umbilical vein (UV) volume blood flow during the second half of normal pregnancy and establish sex-specific reference ranges.

Methods: This work was a prospective longitudinal study of singleton low-risk pregnancies. The UV diameter and maximum blood flow velocity were serially measured by sonography at the intra-abdominal portion of the UV over 19 to 41 weeks. Umbilical vein volume blood flow was calculated and normalized for estimated fetal weight.

Results: One hundred seventy-nine women and their fetuses (87 male and 92 female) were included in the final analysis, and a total of 746 observations were used to construct sex-specific reference intervals. We found no statistically significant sex-specific differences in the UV parameters examined. However, the temporal development patterns of normalized UV volume blood flow appeared to differ between male and female fetuses during the second half of pregnancy, with crossovers at 24 and 32 weeks' gestation.

Conclusions: Umbilical vein volume blood flow is similar among male and female fetuses in quantitative terms, but the pattern of gestational age-dependent temporal changes may be different, which may have important physiologic implications with regard to in utero development and maturation of the fetoplacental unit.