One of the most significant issues when it comes to suspected big babies is that ultrasounds are often wrong and have a margin for error! Simply put there is no accurate method of estimating the size of a baby before birth!
Shoulder dystocia is one of those occasional but rare serious situations.
The theory behind the suggested intervention is that a larger baby is more likely to get stuck at birth. So if we offer inductions for ‘large for dates’ babies, maybe we can prevent most cases of shoulder dystocia from happening in the first place.
Many hospitals will define a large baby as one who weighs more than 4kg at birth, which is 8lb 13oz (locally it is 4.5kg or 9 lb 14.5 oz. However, it is important to know that big babies are normal in well-resourced countries such as the UK and Australia. Healthy, well-nourished women grow healthy well-nourished babies. Genetic factors can also influence the size of babies.
It can’t be argued that, on average, a large baby is more likely to experience shoulder dystocia than a small baby. But most large babies are born without experiencing this. 1:200 babies will experience shoulder dystocia 51% were 4kg or above 49% were below 4kg. Is this a big enough statistical difference to warrant intervention?
Beta et al (2019) showed that only 6% of babies who weighed more than 4kg experienced shoulder dystocia. This means that 94% of babies weighing more than 4kg didn’t experience this. This is for babies who are actually over 4kg, by the way, not those who are predicted as being over 4kg.
To date, no study has been able to accurately predict which babies will experience shoulder dystocia (Leung et al 2011, RCOG 2012a, Boulvain et al 2016) and no study has been able to predict which of these babies will experience significant harm resulting from shoulder dystocia (RCOG 2012a, Boulvain et al 2016).
Estimating babies weight
Estimating the size of a baby by palpating a pregnant person’s bump is incorrect more than 50% of the time (Chauhan et al, 2005).
As with due dates, ultrasound isn’t very accurate at predicting a baby’s size either. Most studies and reviews show that there is a margin of error of 15% either way ( Rossi et al 2013, Boulvain et al 2016, Milner & Arezina 2018). For example, for a baby weighing 8lb at birth the ultrasound estimation could be anywhere between 6 lbs 13oz and 9 lbs 3oz.
A study in America found that two out of three pregnant women were told that their baby was ‘too big’ according to ultrasound. This study found that the average birth weight of the babies estimated to be ‘big’ was 7lb 13oz (Cheng et al 2015).
It is well documented that ‘big’ babies tend to run in families so genetics can also be at play here, Therefore having a big baby can be normal and healthy for that individual. However abnormal blood glucose levels can cause a baby to grow abnormally big due to high blood glucose levels.
If a birthing person has higher than normal blood glucose levels babies who have grown big are a different shape to genetically big babies. Their shoulders and chest tend to be fatter and larger and they are more likely to encounter complications at birth due to this. Therefore if your baby ticks over the 90th centile during pregnancy and before 37 weeks of gestational age. It is usually recommended that you have a gestational diabetes test (FYI, this also comes with its inaccuracies, pros and cons).
Interventions for ‘big babies’
Complications associated with big babies may be due to the interventions carried out when a baby is suspected to be large for gestational age. For example, a study compared the outcomes for women who were suspected of having a big baby, with a group of women who unexpectedly gave birth to a big baby (Sadeh-Mestechkin et al, 2008). Women who were suspected of having a big baby were three times more likely to have an induction of labour or caesarean and were four times more likely to have complications such as perineal tearing and postpartum haemorrhage.
I have said it for a long time - the perceived size of a baby influences outcomes more than the actual size of the baby.
Next steps
A suspected big baby is something that I support parents with often. Do your own reading, ask your healthcare provider questions and make your own informed decision. For example: why are they recommending induction of labour and what evidence is it based on? How likely will the induction of labour end in a vaginal birth without intervention? How likely is the induction of labour likely to end in an unplanned caesarean birth? What are the next steps if we decline intervention?
Use the BRAIN acronym.
You have a voice and you have time.
I am not anti-intervention, far from it. Intervention can save lives but the interventions need to be justified and fully informed. A choice made when both sides have been looked at and taken into consideration. Free from coercion and pressure.
Local handout for a suspected big baby.
Further reading
Why Induction Matters by Rachel Reed
In Your Own Time by Dr Sara Wickham
Both of the above authors also have fabulous websites and blogs.