Evening primrose oil is derived from the Oenothera biennis plant and has been used in a variety of medicinal applications from treating skin conditions to jump-starting labor, explains the U.S. National Library of Medicine's MedlinePlus website. However, due to the lack of substantive evidence and the potential negative effects of evening primrose oil in late pregnancy, it should not be considered for labor induction without consulting with your healthcare provider.
Ripening the Cervix
Evening primrose oil is an herbal remedy that is not capable of causing the cervix to dilate or bring on uterine contractions. Instead, when evening primrose oil is taken orally, it purportedly acts as a prostaglandin, encouraging labor by ripening the cervix, like prostaglandin gel administered by a health care professional or nipple stimulation. Cervical ripening is the softening of the cervix, a process that takes place prior to the onset of labor. The cervix must soften in order for it dilate during labor.
In order to determine whether a woman is a viable candidate for cervical ripening through a medical or natural intervention, the cervix is evaluated by a healthcare professional and assigned a Bishop score. This number is generated from points assigned for a combination of cervical dilation, effacement, firmness, position and the station of the fetus' presenting part. When an evaluation results in a score of 5 or more points, cervical ripening may be considered for medical reasons or postdate pregnancies.
Evidence of Success
A study published in the American Journal of Obstetrics and Gynecology found a substantial difference in the Bishop score in women given evening primrose oil once a day for a period of one week. In comparison to the placebo group's mean Bishop Score of 1.57, the treatment group had a mean score of 3.68. The difference in cervical length between the two groups was an average reduction of 0.89 in the treatment group and only 0.42 in the placebo group. While there was no difference between the groups in the time to the onset of labor, a larger percentage of the treatment group delivered vaginally -- 70 percent versus 51 percent in the placebo group. The results were most prominent in first-time pregnancies and after 39 weeks gestation.
Failure and Risks
A study published in the Journal of Nurse Midwifery concluded there were no differences in length of gestation or length of labor in the treatment or placebo group when evening primrose oil was administered orally from the 37th week onward. The study does, however, caution that the use of evening primrose oil in late pregnancy may be associated with an increased risk of prolonged rupture of membranes, a greater need for oxytocin augmentation in labor, and the fetus' arrested descent.