Objective: To compare resistance to blood flow between two umbilical arteries at the same cord site during the second half of pregnancy.
Methods: We evaluated 80 patients with singleton pregnancies cross-sectionally at gestational ages ranging from 20-40 weeks' gestation. Resistance to blood flow was measured separately by means of systolic-diastolic ratio (S/D) for both umbilical arteries of each subject at the same site of transverse cord section. The higher mean value of the one umbilical artery was designated S/Dmax, whereas the lower mean value of the other paired umbilical artery was designated S/Dmin. The percent difference between the two values was calculated for each pair of measurements. Then these data were stratified by gestational age.
Results: The overall mean (+/- standard deviation [SD]) S/Dmax was significantly different from S/Dmin (2.62 +/- 0.58 versus 2.27 +/- 0.40, respectively P < .001). The mean (+/- SD) calculated percent difference of 14.9 +/- 10.4% ranged in a downward trend over the course of late pregnancy from 29.2 +/- 17.1% in the 20-28 weeks' gestational age group to 10.4 +/-6.1% among those at term (37-40 weeks); the slope of this trend was -1.32 +/- 8.55% per week, a statistically significant trend (P < .001). There was a more than 20% difference in more than one quarter (29%) of the 80 pairs of umbilical arteries we studied. Cases with these large differences were concentrated mostly among those with earlier gestational ages: At term, only 8.6% showed this difference in flow resistance measurements.
Conclusion: The resistance to blood flow in one umbilical artery often differs considerably from that in the other. The difference, which equalizes gradually as pregnancy advances, perhaps as a result of functional maturation of the Hyrtl anastomosis between the vessels, may have clinical importance for identification and evaluation of the potentially jeopardized fetuses, either as an early marker of fetal hypoxia or in interpretating fetal status.