The association between unintended hysterotomy extensions with cesarean delivery and subsequent preterm birth.

BACKGROUND: An increased risk of preterm birth has been observed among individuals with a previous second stage cesarean delivery compared with a previous vaginal delivery. One mechanism that may contribute to the increased risk of preterm birth following second stage cesarean delivery is the increased risk of cervical injury due to extension of the uterine incision (hysterotomy) into the cervix. Investigating the contribution of hysterotomy extension to the rate of preterm birth has not been well-studied and may shed light on the mechanisms underlying the observed relationship between mode of delivery and subsequent preterm birth.

OBJECTIVE: We aimed to quantify the association between unintended hysterotomy extension and preterm birth in a subsequent delivery.

STUDY DESIGN: We performed a retrospective cohort study using electronic perinatal data collected in two university-affiliated obstetrical centers. The study included patients with a primary cesarean delivery of a term, singleton live birth and a subsequent singleton birth in the same catchment (2005-2021). The primary outcome was subsequent preterm birth <37 weeks; secondary outcomes included subsequent preterm birth <34, <32 and <28 weeks. We assessed crude and adjusted associations between unintended hysterotomy extensions and subsequent preterm birth with log binomial regression models using rate ratios (RRs) and 95% confidence intervals (CIs). Adjusted models included several characteristics of the primary cesarean delivery such as maternal age, length of active labour, indication for cesarean delivery, chorioamnionitis, and maternal comorbidity.

RESULTS: A total 4,797 patients met the study inclusion criteria. The overall rate of unintended hysterotomy extension in the primary cesarean delivery was 6.0% and the total rate of preterm birth in the subsequent pregnancy was 4.8%. Patients with an unintended hysterotomy extension were more likely to have a longer duration of active labour, chorioamnionitis, failed vacuum delivery attempt, second stage cesarean delivery, and persistent occiput posterior position of the fetal head in the primary cesarean delivery and higher rates of smoking in the subsequent pregnancy. Multivariable analyses controlling for several confounders showed that a history of hysterotomy extension is not associated with a higher risk of preterm birth <37 weeks gestation (adjusted RR 1.55, 95% CI 0.98-2.47), but is associated with preterm birth <34 weeks gestation (adjusted RR 2.49, 95% CI 1.06-5.42).

CONCLUSION: Patients with a uterine incision extension have a 2.5 times higher rate of preterm birth <34 weeks compared with patients who do not sustain this injury. This association was not observed for preterm birth <37 weeks. Future research should aim to replicate our analyses while incorporating additional data to minimize the potential for residual confounding.