Comparing the Effects of Oral and Vaginal Evening Primrose Capsules with Concurrent Labor Induction and Exclusive Labor Induction on Bishop Score and Some Labor Outcomes: A Triple-Blind Clinical Trial

Background: Cervical ripening and achieving a favorable Bishop score are crucial for a successful delivery. When the cervix is not adequately ripe, exclusive labor induction might not succeed. Therefore, there is a need to identify safe concurrent interventions to improve the Bishop score. Objectives: This study aimed to compare the effects of oral (capsule) and vaginal evening primrose oil, in conjunction with labor induction, on the Bishop score and various labor outcomes in pregnant women. Methods: This randomized, triple-blind clinical trial was conducted on 170 pregnant women admitted to terminate their pregnancies at Niknafs Maternity hospital in Rafsanjan, Iran, in 2020. The subjects were randomly allocated to 2 intervention groups: the oral evening primrose oil group (n = 57) and the vaginal evening primrose oil group (n = 56). A control group (n = 57) received oxytocin induction alone. Group assignment was performed using the minimization method. The first intervention group received two oral evening primrose 1 000 mg capsules; however, the second group received two 1 000 mg vaginal capsules, both in conjunction with labor induction using 10 units of oxytocin per 1 000 cc of Ringer’s serum. The data were collected using a checklist with three parts: The mother’s demographic characteristics, obstetric characteristics, and childbirth details. Descriptive statistics, including frequency, percentage, mean, median, and standard deviation (SD), were employed. Statistical analyses included one-way analysis of variance (ANOVA), independent t-tests (the Kruskal-Wallis test and Mann-Whitney test for non-parametric tests), and the chi-square test. Results: Two hours after induction, the mean Bishop scores in the oral, vaginal, and control groups were 5.47 ± 1.67, 6.28 ± 1.91, and 4.09 ± 1.61, respectively (P < 0.001). Four hours after induction, the respective scores were 7.63 ± 1.59, 8.00 ± 1.20, and 5.18 ± 1.97 (P < 0.001). The vaginal group had significantly higher Bishop scores. Furthermore, the duration of the first and second phases of labor was significantly shorter in the vaginal group than in the oral and control groups (P < 0.001). Conclusions: Vaginal evening primrose oil was significantly more effective than oral capsules and induction with oxytocin alone in terms of the Bishop score and most labor outcomes.