Association between a low umbilical artery pulsatility index and fetal distress in labor in very prolonged pregnancies

Abstract

Objective: To investigate the association between fetal, umbilical and uterine circulatory changes and adverse perinatal findings in very prolonged pregnancies. Study design: 44 women proceeding to 43 completed weeks of gestation with the intention of a trial of vaginal delivery were studied prospectively with ultrasound Doppler velocimetry. An intensified fetal surveillance was routinely commenced at 42 weeks and only uncomplicated pregnancies were allowed to proceed. The endpoint perinatal measures were oligohydramnios, fetal meconium release, fetal distress in labor and birth asphyxia. Flow variables in different groups were compared with the Mann-Whitney U test, Student's unpaired t-test, Wilcoxon signed-rank matched-pairs test. Fisher's exact test and contingency table analysis, and a two-tailed P value < 0.05 was considered statistically significant. Results: The umbilical artery pulsatility index was significantly lower in cases of fetal meconium release ( n = 12) and fetal distress ( n = 7). The umbilical venous flow velocity was significantly lower in cases of meconium, and the fetal aortic volume flow significantly higher in cases of fetal distress. No significant flow changes were found in connection with oligohydramnios ( n = 5) and birth asphyxia ( n = 2). Uterine flow was not significantly affected in any group. Conclusions: In very prolonged pregnancies, fetal distress in labor was not associated with an increased placental vascular resistance. In contrast to previous reports, the umbilical artery pulsatility index was low in cases of fetal distress and meconium release. The etiology is unknown, but a subclinical fetal hypoxia might have triggered a vasodilation of placental vessels. Vasodilation at an unchanged volume flow could also explain the decrease of umbilical venous flow velocity. The increased aortic volume flow indicates an increase of cardiac output in fetuses later developing distress in labor.