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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.

Objectives: Umbilical venous pulsation is an important sign of hemodynamic compromise, but is also found under normal physiological conditions. Mathematical modeling suggests that vascular compliance is a determinant for pulsation, and we tested this by studying velocity pulsation at three sites on the umbilical vein., Methods: In a cross-sectional study of 279 low-risk pregnancies (20-40 weeks' gestational age) blood flow velocity in the umbilical vein was determined before, within and after the umbilical ring in the fetal abdominal wall, and the incidence and magnitude of pulsation (the difference between the maximum and minimum velocity during a pulse, and pulsatility index) were noted. Based on the fact that the vessel cross-sectional area is an important determinant of compliance, we measured the diameter and time-averaged maximum velocity to reflect variation in diameter and compliance at the three sites., Results: The incidence of umbilical venous pulsation was higher at the umbilical ring in the abdominal wall (242/279, 87%, 95% CI 82-90) than in the cord (43/198, 22%, 95%CI 16-27) or intra-abdominally (84/277, 30%, 95% CI 25-36) (P < 0.001). When pulsation was observed intra-abdominally, the pulsatility was not different from that at the umbilical ring (P = 0.16). However, the lowest pulsatility was found in the cord vein (P < 0.0001), where the largest vein diameter was found., Conclusion: The high incidence of venous pulsation at the umbilical ring where diameter and compliance are low supports the suggestion that local compliance is an important factor influencing pulsation in fetal veins., (Copyright 2003 ISUOG. Published by John Wiley & Sons, Ltd.)

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 >or=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 >or=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.)

Objective: To determine the occurrence of high venous velocities at the umbilical ring in the normal early second trimester, based on the assumption that a narrow umbilical ring may cause obstruction and increased venous blood velocity at the abdominal wall., Design: Cross-sectional study., Setting: Hospital antenatal clinic., Population: One hundred and one low risk singleton pregnancies specifically recruited for the study., Methods: Ultrasound was used at 11-19 weeks to determine the diameter and velocity in the umbilical vein at the fetal end of the cord and at the inlet through the abdominal wall. Outcome measures 10th, 50th and 90th centiles were estimated for the time-averaged maximum velocity in the cord and at the abdominal inlet. The increase of velocity as the blood entered the abdominal wall was calculated in percent of the velocity in the cord., Results: During weeks 11-12 there was hardly any difference between blood velocity in the umbilical vein at the umbilical ring and that in the cord. From week 13 onwards it was increasingly common to find blood acceleration at the umbilical ring of 50-500%. Velocity increment >50% was found in 0/12 fetuses (0%) at 11-12 weeks, 5/20 (25%) at 13-14 weeks, and in 21/28 (75%) at 17-19 weeks., Conclusions: Blood velocity is higher in the umbilical vein at the abdominal wall than the cord, particularly after 13 weeks of gestation. If acceleration of blood velocity at the umbilical ring is a sign of a narrow inlet, it seems that a progressive tightening occurs during the second trimester.