Effect of magnesium prophylaxis and preeclampsia on the duration of labor

Our goals were to compare duration of labor at term for (1) women with preeclampsia versus normotensive nulliparous women and (2) nulliparous women with preeclampsia who received magnesium for seizure prophylaxis versus those who did not.

We performed a retrospective cohort study of all nulliparous, term vaginal deliveries from 1989 through 1995 at University of California, San Francisco. The perinatal database and medical records were reviewed for information on duration of labor, maternal and labor characteristics, and neonatal outcomes. The chi2 odds ratio, and Student t tests were used to compare categoric and continuous variables between women with preeclampsia and control women and between women with preeclampsia who did and those who did not receive magnesium. Logistic regression was used to evaluate variables predictive of labor duration.

Our study subjects were 4083 normotensive nulliparous women and 154 women with preeclampsia. A sample size calculation revealed that 1764 normotensive control subjects were needed to show a 10% difference in labor duration with 80% power and alpha of 0.05. Among women with preeclampsia, 93 (60%) were treated with magnesium and 61 (40%) were not. More women with preeclampsia than normotensive women had induction of labor and received epidural anesthesia, prostaglandin gel, and oxytocin (P <.003). Total labor duration did not differ between women with preeclampsia and normotensive women (P =.15) or between women with preeclampsia who received magnesium and those who did not (P =.09). In comparison with normotensive women, those with preeclampsia had a higher rate of postpartum hemorrhage (31% vs 22%, P =.005), and the rate was even higher among preeclamptic women treated with magnesium versus those who received no magnesium (34% vs 26%, P =.002). Logistic regression, with prolonged first stage of labor (>12 hours) used as the outcome variable, indicated that epidural anesthesia (odds ratio 2.3, 95% confidence interval 1.9-2. 6), oxytocin (odds ratio 1.8, 95% confidence interval 1.6-2.2), and persistent occipitoposterior presentation (odds ratio 1.6, 95% confidence interval 1.1-2.4) were associated with prolonged labor, whereas preeclampsia (odds ratio 0.9, 95% confidence interval 0.7-1. 1) and treatment with magnesium were not (odds ratio 1.1, 95% confidence interval 0.9-1.4). Induction (odds ratio 0.5, 95% confidence interval 0.4-0.6) and birth weight <2500 g (odds ratio 0. 5, 95% confidence interval 0.4-0.8) were associated with faster labor.

In term nulliparous women, neither preeclampsia nor magnesium prophylaxis affected labor duration.