In this retrospective study we tested the impact of repeated medical or combined methods of labor induction, and risk of preterm labor in subsequent pregnancy in nulliparous women. Our data indicating that repeated attempts, either medical or combined methods, did not increase the rates of preterm labor in subsequent pregnancy.
Cervical trauma presents an important reason for cervical incompetence, leading to premature delivery. Numerous studies reported comparable results, concluding that mechanical or medical induction of labor10–13 does not increase the risk of premature labor in subsequent pregnancy. Although, these studies addressed the same research question, no data was available to answer the question whether repeated attempts, or combined methods of labor induction, in nulliparous women increase the risk of PTB in a subsequent pregnancy. Furthermore, heterogeneity exists between these studies in term of parity, type of catheter, amount of fluids placed on the catheter, and whether or not traction was applied. It is feasible to postulate that combined methods or repeated medical labor induction have a traumatizing impact on the cervical integrity, preventing the cervix from functioning adequately in future pregnancies, which could lead to spontaneous PTB; such an assumption was not tested before.
Zafran et al.11 pointed out several differences in studies reporting that induction of labor did not increase the rate of premature birth in subsequent pregnancy; among these variations the amount of the fluids placed in the catheter and type of catheter placed in the cervix. Thus too strengthen our study; we included only nulliparous women and double-balloon catheter. In accordance with reports claiming that the amount of fluids inflated into single-balloon catheter plays an important role in the success of cervical ripening,13–15 we placed a high but set amount of 80 cc fluids above and below the internal OS. consideration factors which increase the cervical weakness, it is rational to assume that repeated medical induction attempts or combined methods (medical and mechanical) where a high amount of fluids is placed in the balloon above and below the internal OS, could potentially lead to more cervical tissue injury and therefore have a negative effect on cervical perfection. Our data did not validate this assumption, this approach is safe and did not increase the rate of premature labor in nulliparous in subsequent pregnancy compared to the control group.
Almost 250 nulliparous women were treated with multiple doses of prostaglandin or combined medical and mechanical methods, with no cases of premature labor before 34 weeks observed in this group. The rate of late premature birth between 34-36.6 gestational weeks was 3.1% in the control group, compared to 2.9%, 1.5%, 6.1% 4.6%, respectively, in single medical induction, multiple doses, combined mechanical and medical, and mechanical and mechanical induction groups, respectively. Looking at the group induced by prostaglandin and double-balloon catheter, and the double-balloon induction group, we observed a slightly increased rate of premature labor between 34-36.6 gestational weeks compared to the other groups (Table 2). We assume that this difference could be attributed in part to the shorter pregnancy interval observed in these two groups (Table 1). Previous studies have found that short intervals between the birth of one child and next pregnancy are associated with an increased risk of pre-term birth [16–18]; nevertheless, this slight increase was not statistically significant. Moreover, the fetal secondary outcomes in term of admission to NICU or fetal Apgar score were similar in all groups
In this study, the mechanical group was induced only by double-balloon catheter. In spite of the high amount of fluids placed on the catheter and the traction act, comparable with other studies, we did not observe a significant increased rate of preterm birth. The similarity in preterm birth in all groups, regardless of the induction method, favors the hypothesis that the main mechanism of cervical ripening by the double balloon could be related to prostaglandin release from decidual separation as in single balloon catheters7, rather than by its mechanical effects.
Cesarean delivery was significantly higher in the multiple induction group compared to the control (Table 1). This difference can be attributed to the following: first, we assume that in the combined induction group women were exhausted, as the induction process extended to several days compared to the other groups (3 days compared to 2 days in the mechanical group, one day in the single dose and control group, Table 1). We believe that the women exhaustion played an important role in the decision of delivery mode. Furthermore, this group included high-risk pregnant women who underwent labor induction, which can raise the risk of cesarean section and instrumental delivery.
Although this study has the limitation of being retrospective, it has several strengths. It is the first study testing the hypothesis that pregnant women induced with multiple doses of prostaglandin or with combined methods could have a negative impact on cervical competency, which could lead to premature labor on subsequent pregnancy. This is a large cohort study, using data from one medical center, thus minimizing practice variations in labor induction and delivery protocols. Moreover, we included only nulliparous women to minimize differences among groups.
We conclude that nulliparous women, scheduled for labor induction with multiple prostaglandin doses or combined medical and mechanical methods, do not have increased risk of preterm labor in their subsequent pregnancy.