What is the Big Baby Trial and why are they doing it? 

The Big Baby Trial is being run by the University of Warwick and University Hospitals Coventry and Warwickshire NHS Trust. The researchers obtained over two and a half million pounds (£2,575,726.00) in funding from the National Institute of Health Research(1) to undertake the trial. 

The trial’s website states its purpose as: 

“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.”(2)

What is shoulder dystocia?

The Royal College of Obstetricians and Gynaecologists (RCOG) defines shoulder dystocia as: 

“…a vaginal cephalic delivery that requires additional obstetric manoeuvres to deliver the fetus after the head has delivered and gentle traction has failed.”(3)

There are other definitions used, which makes understanding how common it is quite tricky. The Big Baby Trial website defines shoulder dystocia in two ways – using the RCOG definition, and also as:

“… when the baby’s head has been born but one of the baby’s shoulders becomes stuck behind the woman’s pubic bone (one of the bones in the pelvis), delaying the birth of the baby’s body.”(4)

The problem with the second definition is that what they mean by “delay” is not specified. It is very normal, for instance, for a baby’s head to be born, and the body to not be born until the next tightening happens. During this time the baby will turn (restitution), allowing the shoulders to move through the pelvis, and this can take a few minutes. It can be disconcerting for people who are not expecting this to happen, as the baby’s head will be fully born to the neck, and then nothing more for a while! This is not shoulder dystocia.

Shoulder dystocia happens when (as the second definition does state), the baby’s shoulder is genuinely unable to move past the mother or birthing parent’s pubic bone and the head retracts backwards towards the perineum, and certain interventions are required to free it. This will usually mean asking the woman or person to get into what’s called the McRoberts position – on her/their back with her/their legs pushed backwards. Sometimes she/they may be asked to go onto all fours. Most often, the baby will release itself when the woman or person changes position but sometimes the midwife or doctor will need to attempt to release the shoulder by manoeuvring the baby’s body. Again, usually the baby’s shoulder releases quite quickly, with little or no distress or injury. However, rarely, the shoulder does not release easily, and this quickly becomes an obstetric emergency because the baby’s cord is likely to be compressed, so the baby may not receive sufficient oxygen during the time that their body is still in the birth canal.

Because of this, if the shoulder dystocia does not release easily, intrusive and sometimes painful manoeuvres may include pushing hard onto the birthing woman/person’s abdomen, an episiotomy (cut into the perineum – the area between the vagina and anus), a midwife or doctor putting their hand inside the woman or person’s vagina to try to move the baby into a better position. This may lead to the woman or person feeling that they are being pushed into different positions, or positions that can be very uncomfortable or painful.

How common is shoulder dystocia?

Because there is no centrally agreed definition for shoulder dystocia, it is very difficult to determine exactly how common it is. This is complicated further by the fact that most shoulder dystocias are released easily, or with minimal intervention, so while shoulder dystocia sounds very worrying (and can sometimes be extremely serious) it normally doesn’t cause harm. Yet, these minor shoulder dystocias are normally recorded together with the serious ones rather than being recorded as mild or significant.

Therefore, when looking at shoulder dystocia we need to consider both how often it seems to happen and how often it causes harm.

The Big Baby Trial website states that shoulder dystocia happens in 0.7% of vaginal births, which is about 1/150(4). However, RCOG’s Green Top Guideline states that the figure is between 0.58% (about 1 in 172 births) and 0.7% (the 1/150 as above), so The Big Baby Trial information has chosen to only share the higher figure, which could be considered to be misleading. The RCOG figures(3) are based on a combination of moderate and low grade evidence (2+ and 3) there is still no really high quality research available to show just how common shoulder dystocia really is.

What problems may shoulder dystocia cause?

So, what really matters isn’t how often shoulder dystocia happens, it’s how often the shoulder dystocia leads to injury to the birthing woman or person, and/or the baby. Let’s look at RCOG’s figures(3) again to see what injuries may happen when a shoulder dystocia occurs, and how often they may happen.

Injuries to the mother/birthing parent

The most common injury to the mother or birthing parent is post-partum haemorrhage – or excessive bleeding after birth. This is most likely to be caused by a tear to the perineum or vagina caused by the trauma of the manoeuvres to release the baby. This is estimated to be around 11%(3) of the births which include a shoulder dystocia (not 11% of all births).

Shoulder Dystocia can lead to damage to the birthing woman or person’s perineum and vagina, and of those people whose births included a shoulder dystocia, around 3.8%(3) of them had third or fourth degree tears(5). This is very slightly higher than the rate of third or fourth degree tears which happen in hospital births, which according to RCOG is around 3%(5).

There is also likely to be a higher rate of parents experiencing psychological injuries, such as PTSD, although I have been unable to locate any research specifically on the effects of shoulder dystocia on rates of psychological birth trauma injury. This is because the mental health of birthing women and people is often given far less consideration than physical injuries to the birth parent or baby, even though it is essential to a women’s overall health and wellbeing. It is also worth remembering that not only may parents experience trauma when these types of birth complications happen, but so do the midwives, doctors and other birth attendants. This data is also rarely tracked. Neither is enough focus given to the impact that this trauma has on how midwives and doctors approach birth in the future, and whether their vicarious trauma means that they’re more likely to attempt to encourage pregnant women and people into interventions that aren’t right for them.

Injuries to the baby

The most common injury to the baby is damage to their shoulder, called a brachial plexus injury. RCOG estimates that 2.35% – 16% of babies who have a shoulder dystocia experience a brachial plexus injury, however most babies recover quickly. RCOG states that 1 in 10 of those babies who are actually injured suffer permanent damage. According to RCOG this is around 2 in every 10,000 births. Having said this, brachial plexus injury can happen in a caesarean birth, too – perhaps as high as 3 in every 10,000 caesareans!(6)

The Big Baby Trial’s figures on permanent injury are much higher than RCOG’s. The trial’s website states that 1 in 100 babies who experience a shoulder dystocia will have a permanent shoulder injury(4). There is no explanation given as to why the figures in The Big Baby Trial are so dramatically different to RCOG’s figures.

The biggest fear about shoulder dystocia is the possibility of brain damage or the death of the baby. This is very rare indeed, while of course a devastating tragedy that everyone would like to stop from happening. The website ‘Evidence Based Birth’ discussed the various trials and data on these outcomes in detail(7).

Are bigger babies more likely to experience a shoulder dystocia?

If we look at all of the babies who experience shoulder dystocia, about half of them will be macrosomatic, which means larger than average. However, only a small number of babies are macrosomatic, which means that larger babies are more likely to experience a shoulder dystocia than smaller babies. 

Therefore, The Big Baby Trial is looking at babies who are estimated to be bigger than average. The trial aims to find whether the chance of shoulder dystocia is reduced if labour is induced at 38 weeks rather than waiting for labour to start spontaneously. 

Other than shoulder dystocia, what other outcomes are being looked at?

The Big Baby Trial will also look at whether early induction affects rates of: 

“birth injury, fractures, neonatal asphyxia, maternal haemorrhage, perineal trauma, caesarean section rate, and length of inpatient stay of mother and baby.”(8)

In addition, the trial will ask birthing and non-birthing parents about their experience of the birth, plus it will evaluate the economic impacts of the intervention and outcomes.

How do we know which babies could be included?

Growth scans offered in late pregnancy aim to spot babies who are growing bigger than expected. These are using personalised growth charts which are considered to be more accurate than a scan alone, however they are still frequently wrong! They often lead to many babies incorrectly being estimated to be larger than average. They also miss many babies who are unexpectedly born larger than average. This does mean that there are going to be many people who are included in the trial who have their labour induced, with the harm that comes with induction, who go on to birth babies who are of average or small size. 

How is The Big Baby Trial getting people involved, and what are the concerns with the allocation of women/people to arms of the trial?

Data for The Big Baby Trial is being collected from 75 Trusts across the UK(9). In theory, women and people whose babies are considered to be larger than average will be invited to join the trial, given information on the benefits and risks, and if they choose to go ahead, be allocated to either be induced, or not be induced. Participants should be advised that they can withdraw from the trial at any time(10).

In practice, there is a large amount of anecdotal evidence that women and people are being told that their baby is larger than average and that they will be part of the trial, without being given the information to make an informed decision – or being told that they are the only ones who can decide whether they want to be involved. This means that many women and people are being allocated to either being induced or not induced without understanding that they have the option to not participate.

Some Trusts have been unethically providing prizes to staff who recruit women and people into the trial, which may lead to undue pressure being put on women and people who may not wish to be part of it. Now deleted, this tweet from 2019 is an example of such problems.

What other concerns are there about The Big Baby Trial?

Risk of prematurity 

The Big Baby Trial states that they are looking at whether “labour should be started a little earlier”(10). This sounds benign, until it is considered that those women or people allocated to induction will be induced at 38 weeks. If their normal gestation was going to be 40 or 41 weeks, this means that their baby will be born 3 weeks premature. If their normal gestation was 42 weeks or longer, their baby could be born a month prematurely. Add in that dating scans can be incorrect by up to around 5 days and the trial could lead to babies being born dangerously prematurely. 

Risk of induction 

Induction itself is a risk factor for shoulder dystocia! This is clearly stated on The Big Baby Trial’s website.  

“…having the labour ‘induced’ can increase the chance of shoulder dystocia occurring”(4) 

Appropriate use of funding 

Up for debate is whether running such a huge trial, with its ethical challenges and risks of harm is a good use of £2.5million. Given that other concerns, such as the fact that Black women are five times more likely to die in childbirth than white women, are still not being investigated properly, it must be asked why the focus is on this one area of care. The answer may be in the fact that brachial plexus injuries related to shoulder dystocia related injuries are the third most common reason that people bring legal cases against the NHS, so the economic reasons may be the driver. 

What about the risks of induction?

Parents are feeding back to us that they are very rarely being given impartial and evidence-based information on the risks of induction. Furthermore, the low chance of harm being caused even if a shoulder dystocia happens is also rarely explained. This means that parents don’t have the information that they need to make an informed decision. A paper that has just been published by Dahlen at el(11) compared the outcomes for low-risk births which had started spontaneously and low risk births which were started by induction, and they found:

  • Birth by unplanned caesarean for first time mothers went from 13.8% with spontaneous labour to 29.3% when labour was induced 
  • The use of epidural during labour for first time mothers went from 41.3% with spontaneous labour to 71% when labour was induced 
  • There was an increase in neonatal trauma and babies needing resuscitation when labour was induced 
  • More babies who were induced had respiratory disorders after birth than those who were born after spontaneous labour 
  • Of the babies whose labours were induced, more had health problems up to the age of 16 years, including higher levels of ear, nose and throat infections, respiratory problems and sepsis. 


The Big Baby Trial aims to see whether inducing babies who are considered to be larger than average reduces the rate of shoulder dystocia. While a laudable aim, there are serious concerns around the risks of harm that these early inductions can bring to the picture, as well as the ethics of how the trial is being run. Parents have a right to evidence based, impartial information and currently it appears that few parents are truly given all the information they need to make an informed decision.


  1. National Institute for Health Research Funding information: https://www.fundingawards.nihr.ac.uk/award/16/77/02
  2. Clinical Trials Unit website – Induction of labour for predicted macrosomia – The ‘Big Baby Trial’: https://warwick.ac.uk/fac/sci/med/research/ctu/trials/bigbaby/
  3. RCOG Green Top Guideline number 42 – Shoulder Dystocia: https://www.rcog.org.uk/globalassets/documents/guidelines/gtg_42.pdf
  4. Big Baby Trial website – About shoulder dystocia: https://warwick.ac.uk/fac/sci/med/research/ctu/trials/bigbaby/aboutshoulderdystocia
  5. RCOG Patient Information Leaflet – Care of a third- or fourth-degree tear that occurred during childbirth (OASI): https://www.rcog.org.uk/en/patients/patient-leaflets/third–or-fourth-degree-tear-during-childbirth/#:~:text=A%20third%2D%20or%20fourth%2Ddegree%20tear%20is%20where%20a%20tear,fourth%2Ddegree%20tear%20will%20vary.
  6. Chauhan et al 2014, Neonatal brachial plexus palsy: incidence, prevalence, and temporal trends: https://pubmed.ncbi.nlm.nih.gov/24863027/
  7. Evidence Based Birth – Evidence on: Induction or Cesarean for a Big Baby: https://evidencebasedbirth.com/evidence-for-induction-or-c-section-for-big-baby/
  8. The Big Baby Trial – Information for professionals: https://warwick.ac.uk/fac/sci/med/research/ctu/trials/bigbaby/health
  9. The Big Baby Trial – Trial Updates: https://warwick.ac.uk/fac/sci/med/research/ctu/trials/bigbaby/news
  10. The Big Baby Trial – Information for participants: https://warwick.ac.uk/fac/sci/med/research/ctu/trials/bigbaby/informationforparticipants/