What does a big baby mean?
We say a baby is ‘big’ if it is estimated to be on or above the 95th centile on an ultrasound scan when plotted on your customised GROW 2.0 chart. (National Institute for Health and Care Excellence (NICE) definition).
We calculate your baby’s weight using three measurements - their abdominal (tummy) circumference, head circumference and the length of their femur (thigh bone). We know that there is a 15-20% margin of error when we use ultrasound scans to measure your baby’s growth, but at the moment there is not a more accurate way of measuring your baby. Research also tells us that 58-60% of babies thought to be above the 90th centile are born with a weight that plots on a lower centile . So, there is a high chance that your baby will not be big when it is born.
Why are you worried that my baby is big?
The main thing we are worried about is that your baby may suffer a shoulder dystocia during birth.
What is Shoulder Dystocia?
Shoulder dystocia occurs during vaginal birth when your baby’s head is born, but their shoulder gets stuck behind your pubic bone, requiring midwives or doctors to perform extra manoeuvres to help you give birth. This is thought to happen in around 1 in 150 births (0.7%), we also know that this can happen to babies of any size, not just bigger ones.
Although we have found that shoulder dystocia occurs more often in bigger babies, 94% of babies over 4kg (8lb 13oz) will be born without suffering a shoulder dystocia.
What happens if a baby suffers a shoulder dystocia during birth?
Around 90% of babies who do suffer shoulder dystocia will be born following a simple position change called the McRoberts’s manoeuvre, which involves support to bring your thighs up to your chest while lying flat. Most babies will not suffer long term complications.
In some cases, extra manoeuvres are needed to help you give birth. This can sometimes lead to your baby having temporary or permanent nerve damage, fractured collar bones, and brain damage. In rare cases babies can die.
For more information see RCOG Shoulder Dystocia Leaflet.
What are my options?
Our current practice is to offer you three options for birth:
● Continue your pregnancy and await spontaneous labour
● Induction of labour between 38-39 weeks
● Elective caesarean birth from 39 weeks
What is the evidence?
Induction of labour
The recently published Big Baby Trial (May 2025) aimed to find out if early induction from 38 weeks would reduce the number of births affected by shoulder dystocia.
This study found that there was no statistically significant difference in the incidence of shoulder dystocia in women who had an induction of labour from 38+0 - 38+4 weeks compared to women having standard care.
However, the researchers looked further into the results, by removing the women who naturally gave birth early in the standard care group, the study found that induction of labour between 38+0-38+4 weeks did reduce shoulder dystocia by 1.4%.
What we do know is that when we compare both groups, there was no difference in long term health outcomes for mothers and their babies at 2 and 6 months postpartum.
The study did not cover wider maternal implications of induction for a “big baby”, for example medium and long term physical effects, maternal satisfaction or wider emotional effects including birth trauma.
In the absence of conclusive evidence regarding the safest option for birth, we continue to offer induction of labour from 38-38+4 weeks with the aim of preventing shoulder dystocia.
Caesarean Birth
A caesarean birth is also an option because it completely eliminates the possibility of a shoulder dystocia. This is because your baby is born through an incision in your abdomen rather than coming through the birth canal. We might recommend this to you, particularly if your baby is thought to be over 4.5kg (9lb 14oz) as we know this increases your risk of having a shoulder dystocia. We offer this from 39 weeks of pregnancy because we have found that fewer babies need help with their breathing or admission to the Neonatal Unit when they are born from 39 weeks onwards by caesarean.
It is also worth considering that, as with any operation, caesarean births come with additional risks such as heavy blood loss, infection, blood clots to the legs and lungs and damage to your bladder and bowel. In addition, it can take around six to eight weeks to recover from.
How do I make this decision?
When you decide about waiting, induction of labour or caesarean birth, you need to think about how the risks and benefits of each option feel to you, how they affect your birth preferences and your recovery after birth. You may wish to use the BRAINS tool to help you decide (described below) as this guides you to think about all your options in detail.
You should also refer to our Induction of labour and Elective Caesarean information for more details on those options.
Benefits - What are the benefits of the offered options?
Risks - What are the risks associated with the options and how could they affect my labour and birth?
Alternatives - Are there any alternatives?
Instinct – What do I feel is right and safe for me? What is my gut instinct?
Nothing - What if I decide to do nothing/wait and see? What happens next?
Second Opinion – Can I get a second opinion? Who else can I talk to about this?
What are my next steps?
You do not have to decide how you give birth right away. You have time to go away and think about your options with your family and birth supporters. You can then contact us in the Antenatal Clinic with your decision and/or for further information
Alternatively, if you feel confident in the option that is right for you, please let us know and we can ensure you have everything that you need in place.
If you have any further questions, or wish to discuss this further, please contact Antenatal Clinic on 01270 612171.
