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Big Baby

Should labour be started a little earlier for women with babies who appear to be bigger than expected for their dates?

The purpose of the Big Baby trial is 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.

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Listen to Professor Siobhan Quenby talking to Dr Mark Porter about the Big Baby Trial - BBC 4 - Inside Health 

Key information


Induction of labour for predicted macrosomia - The 'Big Baby' Trial

Chief Investigators:

Professor Siobhan Quenby - Warwick Clinical Trial Unit, The University of Warwick and UHCW

Professor Jason Gardosi - 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


Difficulty in delivering the shoulders, after a baby’s head has been delivered, can be a serious complication during a vaginal birth. In most cases where this happens, the baby is larger than average. It has been suggested that if we can predict, by ultrasound scan in the last weeks of pregnancy, which babies are particularly large and at the highest risk of shoulder dystocia, then we could deliver them a week or two earlier by inducing labour, which would reduce the chance of this complication.

It is not clear whether it is better for women and their babies to have labour induced early or to wait for labour to begin naturally. To find out whether delivering large babies earlier is the right thing to do for the baby and mother, a clinical trial is needed. We propose to study 4,000 pregnant women whose ultrasound scans suggests that their babies are particularly large and may be at risk of difficulty with delivery of the shoulders during birth.

With the woman’s consent, she will be allocated at random into either an early induction of labour group, with the aim to give birth at, or soon after, 38 weeks, or a control group where care is as usual and the start of labour is awaited, unless it becomes necessary to induce labour after 38 weeks. 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 difficulty with the delivery of the baby’s shoulders. We will also assess the benefits and risks to women and their babies in each group.

This trial will help decide what the safest method is to care for pregnant women where, because of the large size of the baby, complications may occur during labour and birth.

There will also be a parallel Cohort Study of 3000 women who decline to be randomised into the Randomised Controlled Trial (RCT) but would still like to take part in the research.

Sample size:

4000 women identified as having large for gestational age pregnancies in the RCT.

3000 women identified as having a large for gestational age pregnancies who decline randomisation into the RCT will be allocated to the parallel Cohort Study.

Primary outcome:

The incidence of shoulder dystocia is the primary outcome, with assessment of the validity of the diagnosis, for all randomised women, by an independent expert group.


42 months: 01/01/2018 - 30/06/2021.


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