Induction of labour versus standard care to prevent shoulder dystocia in fetuses suspected to be large for gestational age in the UK (the Big Baby trial): a multicentre, open-label, randomised controlled trial

Abstract

Background: The benefits and harms of early induction of labour to reduce shoulder dystocia in fetuses suspected to be large for gestational age (LGA) are uncertain. We aimed to investigate whether early induction of labour is associated with a reduced risk of shoulder dystocia compared with standard care.

Methods: In this open-label, randomised controlled phase 3 trial, women aged ≥18 years with a suspected LGA fetus (estimated fetal weight >90th customised percentile) as identified by ultrasound scan between 35 weeks and 0 days (35⁺⁰ weeks) of gestation and 38⁺⁰ weeks' gestation, recruited from 106 hospitals across England, Scotland, and Wales in the UK, were randomly assigned (1:1) by web app to standard care or induction of labour between 38⁺⁰ weeks' gestation and 38⁺⁴ weeks' gestation using minimisation, balancing site, estimated fetal weight percentile (≤95th EFW percentile or >95th EFW percentile), and maternal age (≤35 years or >35 years). Key exclusion criteria included drug-treated diabetes, gestational diabetes, and elective caesarean section or induction already planned or indicated for any reason. Our primary outcome was incidence of shoulder dystocia, assessed by a masked independent expert adjudication panel who reviewed participants' delivery notes. Induction of labour was anticipated to result in birth 10·5 days earlier with a 300 g lower birthweight on average than standard care. We did an intention-to-treat (ITT) analysis in all participants for whom we had primary outcome data, and a per-protocol analysis in participants in the induction group who went into labour or were induced at 38⁺⁰ to 38⁺⁴ weeks' gestation versus participants in the standard care group who had not started labour, been induced, or had an elective caesarean section before 38⁺⁴ weeks' gestation. This study was registered with ISRCTN (18229892) and is no longer recruiting.

Findings: Between June 8, 2018, and Oct 25, 2022, 2893 women were randomly assigned to induction of labour (n=1447) or standard care (n=1446); the trial was terminated before the target of 4000 participants was reached on advice of the data monitoring committee following the lower-than-expected incidence of shoulder dystocia in the standard care group. Two participants in the induction group and seven in the standard care group had missing data for the primary outcome and were excluded from the ITT analysis. In the ITT analysis, 33 (2·3%) of 1445 babies in the induction group versus 44 (3·1%) of 1439 in the standard care group had shoulder dystocia (risk ratio [RR] 0·75 [95% CI 0·51-1·09]; p=0·14) with a mean difference of -6·0 days' (95% CI -6·3 to -5·6) gestation and -163·6 g (-190·0 to -137·1) birthweight between trial groups. 355 (24·6%) of 1446 mothers in the standard care group were induced, delivered, or went into labour at or before 38⁺⁴ weeks' gestation. In the per-protocol analysis, 27 (2·3%) of 1180 babies in the induction group versus 40 (3·7%) of 1074 in the standard care group had shoulder dystocia (RR 0·62 [0·41-0·92]; p=0·019), and there was a mean difference of -8·1 days' (-8·4 to -7·9) gestation and -213·3 g (-242·0 to -184·6) birthweight between trial groups. One neonatal death occurred from perinatal asphyxia after shoulder dystocia in the standard care group, and one neonatal death occurred following sepsis and congenital pneumonia in the induction group. 88 (6·1%) of 1447 mothers in the induction group had an adverse event versus 108 (7·5%) of 1446 in the standard care group (RR 0·81 [0·62 to 1·06]; p=0·13). Similar numbers of serious adverse events were reported in both groups.

Interpretation: No significant difference in incidence of shoulder dystocia was found between trial groups in the ITT analysis, probably due to the high proportion of earlier-than-expected deliveries in the standard care group reducing the intended between-group differences in gestational age and birthweight. However, in the per-protocol analysis, compared with all deliveries after 38⁺⁴ weeks' gestation, induction of labour between 38⁺⁰ weeks' gestation and 38⁺⁴ weeks' gestation did show a significant reduction in shoulder dystocia. This study provides pregnant women with suspected LGA fetuses and their clinicians important information about choices and decision making for timing and mode of birth.

Funding: National Institute for Health and Care Research Health Technology Assessment Programme.