Induction of labour for predicted macrosomia: ‘The Big Baby’ randomised controlled trial and economic evaluation

Large-for-gestational age fetuses have an increased risk of shoulder dystocia. This can lead to fractures, brachial plexus injury, hypoxic ischaemic encephalopathy and fetal death. Early induction of labour in women with a fetus suspected to be macrosomic may mitigate the risk of shoulder dystocia as the fetus would weigh less than at 40 weeks gestation. The Big Baby Trial aims to find if induction of labour at 38⁺⁰-38⁺⁴ weeks gestation, in pregnancies with a suspected large-for-gestational age fetuses, reduces the incidence of shoulder dystocia.

The Big Baby Trial is a multicentre, prospective, individually randomised controlled trial of induction of labour at 38⁺⁰ to 38⁺⁴ weeks gestation versus standard care, of fetuses that have an estimated fetal weight >90^th customised centile according to ultrasound scan at 35⁺⁰ to 38⁺⁰ weeks gestation. There is a parallel cohort study for women who decline randomisation because they opt for induction, expectant management or caesarean section. Up to 4,000 women will be recruited and randomised to either induction of labour or to standard care.

The primary outcome is the incidence of shoulder dystocia; assessed by an independent expert group, blind to treatment allocation, from delivery records. Secondary outcomes include birth trauma, fractures, haemorrhage, caesarean section rate and length of hospital stay. The main trial ran seamlessly following an internal pilot study. We are conducting a health economic evaluation of induction of labour at 38⁺⁰ to 38⁺⁴ weeks gestation in this patient population as part of the Big Baby Trial.