Effect of magnesium prophylaxis and preeclampsia on the duration of labor

OBJECTIVE: Our goals were to compare duration of labor at term for (1) wome n with preeclampsia versus normotensive nulliparous women and (2) nulliparo us women with preeclampsia who received magnesium for seizure prophylaxis v ersus those who did not. STUDY DESIGN: 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 inf ormation on duration of labor, maternal and labor characteristics, and neon atal outcomes. The chi(2) odds ratio, and Student t tests were used to comp are 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. RESULTS: Our study subjects were 4083 normotensive nulliparous women and 15 4 women with preeclampsia. A sample size calculation revealed that 1764 nor motensive control subjects were needed to show a 10% difference in labor du ration 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 pr eeclampsia than normotensive women had induction of labor and received epid ural anesthesia, prostaglandin gel, and oxytocin (P<.003). Total labor dura tion 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 pre eclampsia had a higher rate of postpartum hemorrhage (31% vs 22%, P=.005), and the rate was even higher among preeclamptic women treated with magnesiu m versus those who received no magnesium (34% vs 26%, P=.002). Logistic reg ression, with prolonged first stage of labor (>12 hours) used as the outcom e variable, indicated that epidural anesthesia (odds ratio 2.3, 95% confide nce 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 treatmen t 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 wei ght <2500 g (odds ratio 0.5, 95% confidence interval 0.4-0.8) were associat ed with faster labor. CONCLUSIONS: In term nulliparous women, neither preeclampsia nor magnesium prophylaxis affected labor duration.