Studies have found that mifepristone (Mifeprex), a progesterone antagonist, may soften the cervix and cause uterine contractions. This medication has been shown to be effective for elective abortions and medical termination of pregnancy during the first trimester, leading to studies of the effect of mifepristone in term pregnancies. Results of these studies have demonstrated that mifepristone may ripen the cervix and induce labor while not increasing the risk to the fetus. However, most of these studies were small, and the dosing schedules were varied. Berkane and colleagues evaluated the effectiveness of mifepristone in ripening the cervix and inducing labor in term pregnancies.
The prospective double-blind, placebocontrolled study included patients at multiple medical centers. The inclusion criteria were a singleton pregnancy between 37 and 41 weeks of gestation plus three days, a medical indication for delivery within 48 hours, and an unripe cervix (Bishop score of 4 or less). Patients were excluded if they had a contraindication to labor, were undergoing steroid therapy, had renal failure or diabetes, or underwent aspirin or nonsteroidal anti-inflammatory drug therapy within the previous 15 days. Participants were randomized to receive 50, 100, 200, 400, or 600 mg of mifepristone or placebo. Maternal and umbilical arterial blood samples were drawn for mifepristone assay. Umbilical vein cord blood was assayed for adrenocorticotropin hormone and cortisol levels. Effectiveness of mifepristone was defined as labor between 12 and 45/54 hours from the start of treatment. Labor was defined as uterine contractions that gradually changed the cervix or a Bishop score of 6 or higher. Those who reached a Bishop score of 6 without labor had an amniotomy and oxytocin infusion. Maternal and fetal tolerability of mifepristone were assessed.
The study included 346 patients. There were no significant differences between the treatment groups in demographics or medical and obstetric history. The main indications for labor induction were post-term, hypertension, fetal disease (e.g., small for gestational age or macrosomia), premature rupture of membranes, polyhydramnios, or oligohydramnios. The mifepristone treatment was successful in 52.7 percent of patients with no significant difference across the labor induction groups. The rate of vaginal deliveries and fetal tolerability also were similar in all of the study groups. Four serious maternal adverse events occurred—three patients with uterine hypertony and one with maternal bradycardia. No significant differences between the various groups were found when the incidence of acute fetal distress was analyzed. Labor was prolonged in the groups that received 400 and 600 mg of mifepristone compared with those receiving lower dosages.
The authors conclude that up to 600 mg of mifepristone does not induce labor in patients with unfavorable cervical status. They add that the failure of mifepristone in this study may be secondary to the conditions in which it was used. Other studies are needed to determine whether mifepristone effectively induces labor.