The purpose of this study was to compare the efficacy and safety profile of prostaglandin E₂ with isosorbide mononitrate for cervical ripening before the induction of labor at term.

Primigravid women were assigned randomly to receive either 40 mg of isosorbide mononitrate or 2 mg of prostaglandin E₂. Efficacy outcomes were the cervical ripening effect of each agent and the time from treatment initiation to delivery. Safety outcomes were the incidence and frequency of maternal side effects and events that would be potentially hazardous for mother and baby during outpatient cervical ripening.

Prostaglandin E₂ was more effective than isosorbide mononitrate in inducing a change in modified Bishop score. Mean duration from treatment initiation to delivery was greater for isosorbide mononitrate than prostaglandin E₂. There were no adverse events in the isosorbide mononitrate group that would contraindicate outpatient treatment. However, in the prostaglandin E₂ group, 7% of the pregnancies had abnormal fetal heart rate patterns (P = .0002). Maternal satisfaction was significantly higher in the isosorbide mononitrate group.

Although isosorbide mononitrate was less effective, maternal satisfaction was significantly greater. The safety profile of each agent was such that it would be reasonable to give isosorbide mononitrate, but not prostaglandin E₂, on an outpatient basis.