Big Baby
Induction of labour for predicted macrosomia -The 'Big Baby Trial'
The purpose of the Big Baby Trial was to find out if starting labour earlier than usual, at 38 weeks, makes it less likely that shoulder dystocia will happen in women whose babies appear to be bigger than expected.
The trial is now complete and the results have been publishedLink opens in a new window.
Study Summary:
Shoulder dystocia is a complication that occurs in 1 in 150 vaginal deliveries. This is when after a baby's head has been born one of the baby's shoulders becomes stuck behind the woman's pubic bone, delaying the birth of the baby's body. Most babies born that have experienced shoulder dystocia will have no long term complications. But for some babies this can cause a stretching in the nerves of the neck, which may cause long-term weakness in the arm. We know that shoulder dystocia occurs more often in bigger babies but there is uncertainty in how often this actually occurs. We estimate that shoulder dystocia could occur in 1 in 25 women with a big baby who have a vaginal delivery.
It has been suggested that if we can deliver bigger babies a few weeks early by inducing labour, this could reduce the chance of shoulder dystocia happening. Currently it is not clear whether it is better for women with big babies to have their labour induced or to wait for labour to begin naturally. To answer this question a clinical trial is needed. We propose to study 4,000 pregnant women whose ultrasound scans suggests that their babies are bigger than expected.
With the woman’s consent, she will be allocated at random to either have an induction of labour at 38 weeks or to wait until labour starts naturally. We will compare outcomes between the two groups to look at whether, as a result of induction of labour at about 38 weeks, there were fewer complications such as shoulder dystocia. There will also be a parallel Cohort Study of women who decline to be randomised (for example if the woman requests a Caesarean section) but would consent for us to study their delivery information.
This trial will help women, obstetricians and midwives decide what is the best way to deliver bigger babies.
The trial is being led by a partnership between University of Warwick, University Hospitals Coventry and Warwickshire (UHCW) NHS Trust and the Perinatal Institute.
Sample Size:
The randomised controlled trial aimed to recruit 4000 women identified as having large for gestational age babies between 35 and 38 weeks gestation. It was estimated that 3000 women would be recruited into the parallel cohort study.
Primary outcome:
The primary outcome is shoulder dystocia. This is defined as 'a vaginal cephalic delivery that requires additional obstetric manoeuvres to deliver the fetus after the head has delivered and gentle traction has failed'. An assessment of validity of the diagnosis for all randomised women was undertaken by an independent expert panel.
Trial Results:
Trial Results
The trial has been published in The Lancet on 1 May 2025. See link: https://doi.org/10.1016/S0140-6736(25)00162-XLink opens in a new window
The study has provided important evidence for clinical management options that increases choices for women with large babies, to choose a birth plan that can include:
- Waiting for labour to start naturally without worrying about increasing risk to babies if delivery occurs on a labour ward with highly trained staff;
- Choosing a Caesarean section, or
- Induction at 38 weeks to reduce the risk of the baby’s shoulder being caught, without having to worry that this increases the risk of emergency Caesarean section or mother’s tearing during delivery
Health Economic Analysis
The health economics results have been published in the British Journal of Obstetrics and Gynaecology on 1 May 2025. See link: https://doi.org/10.1111/1471-0528.18160Link opens in a new window
The study provided important evidence on the cost-effectiveness of early induction for women with large babies. We found:
- Early induction of labour increased neonatal costs.
- It was not cost-effective when effects are restricted to maternal quality-adjusted life years (QALYs).
- Planned caesarean section might be cost-saving when compared to early induction.
- The study was not able to incorporate any long-term effects on mother and/or infant.
Latest News:
On 1st May, the BigBaby trial results were published in The Lancet.
Professor Siobhan Quenby, BigBaby Co-Chief Investigator, has been awarded an MBE for her ground-breaking research in obstetrics as part of the 2025 New Year’s Honours List.
The BigBaby trial was helped substantially by patient and public involvement, led by the Erb's Palsy Group. Their report is available hereLink opens in a new window.
Chief Investigators:
Professor Siobhan Quenby, Warwick University and University Hospitals Coventry and Warwickshire
Professor Jason Gardosi, The Perinatal Institute
Sponsor:
University Hospitals Coventry and Warwickshire NHS Trust
Funder:
This project was funded by the National Institute for Health Research Health Technology Assessment Programme (project number 16/77/02)
Registration Number:
ISRCTN18229892
Contact Us:
If you have questions about the Big Baby trial, you can contact the trial team at the Warwick Clinical Trials Unit:
Email:Bigbaby@warwick.ac.uk
