Optimal timing for induction of labor in normotensive women: A retrospective cohort study

Labor induction is offered to reduce the risk of stillbirth at late term (41⁺⁰ to 41⁺⁶) but earlier induction in normotensive singleton pregnancies is supported by weak evidence. The aim of the present study was to investigate the optimal timing for induction in normotensive women.

This was a retrospective cohort study including 70 293 singleton term births in Iceland during 1997-2018. Women with serious pre-gestational comorbidity, hypertension, stillbirth or previous cesarean birth were excluded. The risk of adverse maternal and neonatal outcomes for induction at each week from gestational age 37-41 weeks was compared with expectant management, defined as deliveries at a later gestational age. Risk ratios (RRs) and 95% confidence intervals (95% CI) were calculated using log-binomial regression adjusting for sociodemographics, parity, and pregnancy complications.

The risk of cesarean was lower with induction at ≥40⁺⁰ than expectant management, especially at late term (RR 0.73, 95% CI: 0.63-0.83). Respiratory distress was diagnosed in 4.4% of infants after induction from 37⁺⁰ to 37⁺⁶ but 1.3% in the expectant management group (RR 3.08, 95% CI: 1.97-4.81). Induction between 37⁺⁰ and 38⁺⁶ compared with expectant management was associated with a reduced risk of shoulder dystocia, but this was non-significant in births of infants with normal birthweight.

Labor induction from 40⁺⁰ compared with expectant management was associated with reduced risk of cesarean birth in Icelandic women without an increase in risk of adverse maternal or neonatal outcomes. No additional benefit appeared to be from inducing at earlier gestations in low-risk pregnancies.