Acidemia at Birth, Related to Obstetric Characteristics and to Oxytocin Use, During the Last Two Hours of Labor

Abstract

During the active phase of the second stage of labor, with maternal expulsive efforts, there is a decline in fetal pH values and an increase in lactic acid concentrations. Little information is available on the association between the occurrence of acidemia at birth and oxytocin use during the second stage of labor. This case-control study compares uterine contraction frequency, oxytocin use, and other obstetric parameters during the last 2 hours of labor in neonates with acidemia (umbilical artery pH <7.05) and without acidemia (cord artery pH ≥7.05) at birth. Cardiotocography tracings were recorded in a blinded fashion. A hyperactive contraction pattern was defined as 6 or more contractions/10 minutes. A total of 304 neonates with acidemia were identified among 28,486 singleton newborns ≥34-weeks’ gestational age delivered at 2 university hospitals in Sweden between 1994 and 2004. Baseline neonatal and obstetric characteristics were similar in the cases and controls. Oxytocin administration during the last 2 hours of labor was more common in cases compared with controls (61.2% vs. 41.7%, P < 0.001). Risk factors associated with acidemia at birth in univariate analysis were ≥6 contractions/10 minutes [odds ratio (OR) 4.94 with a 95% confidence interval (CI) of 3.25–7.49, P < 0.001]; oxytocin use (OR 2.20 with a 95% CI of 1.66–2.92, P < 0.001); bearing down ≥45 minutes (OR 1.77 with a 95% CI of 1.31–2.38, P < 0.001); and occipito-posterior position (OR 2.18 with a 95% CI of 1.19–3.98, P < 0.01). Multivariate logistic regression analysis showed that only ≥6 contractions/10 minutes (OR 5.36 with a 95% CI of 3.32–8.65) and oxytocin use (OR 1.89 with a 95% CI of 1.21–2.97) were independent risk factors for acidemia. Of the cases with hyperactive labor, 75% had received oxytocin. Cardiotocography patterns were abnormal in 68.8% of cases and in 26.1% of controls (P < 0.001). These findings are consistent with previous reports showing an association between hyperactive uterine contraction patterns and fetal acidemia. The investigators believe that this is the first study to demonstrate that oxytocin use is a risk factor for acidemia after adjustment for the influence of uterine contraction frequency.