Uterine contractile activity and neonatal outcome - A blind analysis of a randomized controlled trial cohort

Sufficient contractions are necessary for a successful delivery but each contraction temporarily constricts the oxygenated blood flow to the fetus. Individual fetal or placental characteristics determine how the fetus can withstand this temporary low oxygen saturation. However, only a few studies have examined the impact of uterine activity on neonatal outcome and even less attention has been paid to parturients' individual characteristics. Our objective was therefore to find out whether fetuses compromised by maternal or intrapartum risk factors are more vulnerable to excessive uterine activity.

Uterine contractile activity was assessed by intrauterine pressure catheters. Women (n = 625) with term singleton pregnancies and fetus in cephalic presentation were included in this secondary, blind analysis of a randomized controlled trial cohort. Intrauterine pressure as Montevideo units (MVU), contraction frequency/10 min and uterine baseline tone were calculated for 4 h prior to birth or the decision to perform cesarean section. Uterine activity in relation to umbilical artery pH linearly or ≤7.10 was used as the primary outcome. Need for operative delivery (either cesarean section or vacuum-assisted delivery) due to fetal distress was analyzed as a secondary outcome. In addition, belonging to vulnerable subgroups with, for example, chorioamnionitis, hypertensive or diabetic disorders, maternal smoking or neonatal birthweight <10th percentile were investigated as additional risk factors.

A linear decline in umbilical artery pH was seen with increasing intrauterine pressure in all deliveries (p < 0.001). Among parturients with suspected chorioamnionitis, every increasing 10 MVUs increased the likelihood of umbilical artery pH ≤7.10 (odds ratio [OR] 1.17, 95% confidence interval [CI] 1.02-1.34, p = 0.023). The need for operative delivery due to fetal distress was increased among all laboring women by every increasing 10 MVUs (OR 1.05, 95% CI 1.01-1.09, p = 0.015). This association with operative deliveries was further increased among parturients with hypertensive disorders (OR 1.23, 95% CI 1.05-1.43, p = 0.009) and among those with diabetic disorders (OR 1.13, 95% CI 1.04-1.28, p = 0.003).

Increasing intrauterine pressure impairs umbilical artery pH especially among parturients with suspected chorioamnionitis. Fetuses in pregnancies affected by chorioamnionitis, hypertensive or diabetic disorders are more vulnerable to high intrauterine pressure.