Inter-pregnancy interval and uterine rupture during a trial of labour after one previous caesarean delivery and no previous vaginal births: a retrospective population-based cohort study.

Background: Short interpregnancy interval (IPI) following caesarean delivery is associated with uterine rupture in subsequent pregnancies. However, the interval required to minimise this risk is unknown. We investigated how the interval between pregnancies and induction or augmentation of labour affect the likelihood of uterine rupture among parturients with one previous livebirth by caesarean delivery who had a subsequent trial of labour. Methods: In this population-based cohort study, we used data from U.S National Vital Statistics System from 2011 to 2021. Multiple pregnancies and births of infants with congenital abnormalities were excluded. A linear spline logistic regression with one knot was used to assess the relationship between uterine rupture and interpregnancy interval for spontaneous and for induced/augmented labours. Multivariable logistic regression was performed with multiple imputation and stepwise backward elimination to adjust for maternal demographic and clinical factors including maternal age, height, and BMI and gestational age. The predicted risk of uterine rupture was tabulated for interpregnancy intervals between zero and 21 months. Adverse outcomes were compared between labours with and without uterine rupture. Findings: We examined 491,998 trials of labour among parturients with one previous livebirth by caesarean delivery and no previous vaginal births. The odds ratio (OR) of uterine rupture per three months interpregnancy interval was 0.91 (95% CI 0.88-0.94) between zero and 21 months after adjusting for confounders, with no further change in risk detected beyond 21 months. The OR was 2.51 (95% CI 2.27-2.78) for induced or augmented labours compared with spontaneous labours. Other factors associated with uterine rupture included older maternal age, shorter maternal height, more advanced gestational age (from 35 to 43 weeks), and heavier birthweight. Predicted rates of uterine rupture ranged from 0.36% at zero to 0.19% at 21 months' interpregnancy interval for spontaneous labours and from 0.91% to 0.47% for induced/augmented labours for parturients with a typical clinical and demographic background. When uterine rupture occurred, the rates of unplanned hysterectomy, intrapartum or neonatal death, and neonatal seizures were 4.0% (95% CI 3.2-5.1%), 3.7% (95% CI 2.7-5.1%), and 2.6% (95% CI 1.8-3.3%) respectively. Interpretation: The risk of uterine rupture progressively decreases as IPI increases until about 21 months and then stabilises. Counselling should advise that for women choosing between a planned TOLAC or a planned caesarean delivery after one previous caesarean delivery and no previous vaginal births waiting until 21 months or longer after a prior low transverse caesarean delivery might minimise the risk of uterine rupture. The absolute risk of certain serious maternal and fetal/neonatal complications such as unplanned hysterectomy and perinatal death is low. Funding: No funding.