Relationship of umbilical vein blood flow to growth parameters in the human fetus

OBJECTIVE: Our purposes were to determine the relationship of the growth of umbilical blood flow to growth in body measurements of human fetuses in uncomplicated pregnancies. The study also aimed to assess the relative contributions of growth in umbilical vein diameter and of increased velocity to the increase in umbilical blood flow. STUDY DESIGN: An animal study was conducted to assess the accuracy of umbilical vein blood flow measurements obtained by triplex mode ultrasonography. Seven pregnant ewes underwent triplex mode umbilical vein flow determination. These results were compared with historical flow data obtained by a steady-state diffusion technique in 34 ewes matched for gestational age and weight. In a separate study performed on human beings, reproducibility and precision of triplex mode flow determination were assessed, as were the relationships between umbilical vein flow and gestational age and head and abdominal circumferences. This cross- sectional study was performed with 70 healthy fetuses ranging from 20 weeks' gestation to term. Best-fit interpolating equations and confidence limits were calculated for blood flow measurements versus gestational age and head and abdominal circumferences. RESULTS: In the validation study performed on sheep there were no significant differences between triplex mode and steady- state measurement groups with respect to gestational age or weight. The umbilical vein flows were similar between triplex mode and steady-state measurement groups (P = .881). In the human study the intraobserver and interobserver coefficients of variation for the vein diameter, mean velocity, and absolute umbilical vein blood flow varied from 2.9% to 12.7%. The mean duration of examination was 3±1 minutes. The umbilical vein diameter and mean velocity increased throughout pregnancy. The absolute umbilical vein flow increased exponentially from 97.3 mL/min at midgestation to 529.1 mL/min at 38 weeks' gestation, whereas umbilical vein flow per kilogram of fetal weight did not change significantly with gestational age. There was a strong correlation between absolute umbilical vein flow and the fetal head and abdominal circumferences. CONCLUSIONS: The triplex mode ultrasonographic technique can play an innovative role in obtaining quick and reproducible measurements of umbilical vein blood flow. The approach was validated with a sheep model. Umbilical vein blood normalized for fetal weight (milliliters per minute per kilogram of fetal weight) and absolute flow (in milliliters per minute) are consistent with previous human studies. We have established new reference values of umbilical vein blood flow relative to head and abdominal circumferences. The growth of umbilical venous diameter accounted for most of the growth in umbilical vein flow.