The association between induction of labour in nulliparous women at term and subsequent spontaneous preterm birth: a retrospective cohort study.

Objectives: To evaluate the rate of subsequent spontaneous preterm birth in patients with previous induction of labour at term compared to women with previous spontaneous labour at term.

Methods: This was a retrospective cohort study of all women with consecutive births at the Royal Brisbane and Women's Hospital between 2014 and 2018. All nulliparous women with a singleton pregnancy and induction of labour at term or in spontaneous labour at term in the index pregnancy were included. Data was extracted from electronic medical records. The outcome of spontaneous preterm birth in the subsequent pregnancy was compared between patients with previous term induction of labour and in previous term spontaneous labour.

Results: A total of 907 patients with consecutive births met the inclusion criteria; of which 269 (29.7%) had a term induction of labour and 638 (70.3%) had a term spontaneous labour in the index pregnancy. The overall subsequent spontaneous preterm birth rate was 2.3%. Nulliparous women who underwent term induction of labour were less likely to have a subsequent preterm birth compared to nulliparous women in term spontaneous labour (0.74 vs. 2.98%; odds ratio [OR], 0.25; 95% confidence interval, 0.06-1.07; p=0.0496) in the index pregnancy. This however was not significant once adjusted for confounders (adjusted OR, 0.29; p=0.10). Spontaneous preterm birth was associated with a previous spontaneous labour compared to induction of labour between 37 to 37 and 38 to 38 weeks (adjusted OR 0.18 and 0.21; p=0.02 and 0.004 respectively).

Conclusions: Term induction of labour does not increase the risk of subsequent spontaneous preterm birth compared to spontaneous labour at term in nulliparous women. Further research is needed to validate these findings in a larger cohort of women and to evaluate the effect of elective IOL among low-risk nulliparous women.