Background: Umbilical vein constriction at the fetal abdominal inlet is a common finding after week 13, when the period of umbilical herniation is brought to an end., Aims: To test the hypothesis that a constricting umbilical ring within physiological ranges affects fetal hemodynamics by either pooling blood in the placenta or restricting nutrient transfer to the fetus and thus shift the birthweight/placental weight (BW/PW) ratio. A constriction could also cause pressure changes and elongation of the cord and possibly be a disadvantage during labour., Study Design: Cross-sectional., Subjects: 359 Low-risk singleton pregnancies at 13-40 weeks of gestation., Outcome Measures: Standard deviation score (z-score) and regression analysis were used to determine the effect of umbilical vein constriction (expressed by increased blood velocity) on birthweight/placental weight ratio (BW/PW), cord length, Apgar score and emergency delivery due to fetal distress., Results: Umbilical venous constriction had a mild but significant effect on BW/PW in male (p=0.018) but not in female fetuses. Increased constriction was also associated with increased length of the cord but only in female fetuses (p=0.019). Cord length was positively related to birthweight and placental weight, but an increased length of the cord was also associated with decreasing BW/PW ratio for the male fetuses only (p=0.044). Increasing degree of venous constriction was associated with Apgar score < or =7 at 1 (p=0.009) but not at 5 min after birth and was not associated with emergency delivery., Conclusion: Physiological umbilical venous constriction exerts a mild but significant gender-specific hemodynamic impact on intrauterine development.