Updated: June 2023
Big babies are normal in well resourced countries. Over 10% of babies born in the UK and Australia weigh 4kg (8lb 13oz) or more. Healthy well nourished women grow healthy well nourished babies. Genetic factors also influence the size of babies (big babies run in families); and each baby a woman has usually weighs more than the last. Babies also continue to grow at the end of pregnancy (because placentas continue to nourish them rather than switch off) – so a baby will be bigger at 42 weeks than they were at 40 weeks.
However, abnormal blood glucose levels (BGLs) – with uncontrolled gestational diabetes (GD) – can also cause a baby to grow big. Babies who are big because of high BGLs are a different shape to ‘normally’ large babies. In particular, their shoulders and chest are larger and fatter, and they are more likely to encounter complications at birth. Unfortunately, research into big babies usually combines the outcomes for GD babies with non-GD babies.
The only way to accurately assess the weight of a baby is to weigh them after birth. Clinical assessment ie. palpating and measuring pregnant bumps is incorrect more than 50% of the time (Chauhan et al. 2005). Measuring the baby’s abdomen with an ultrasound – only predicts the weight of the baby within 15% of their actual weight (Rossi et al. 2013). Around half of the babies estimated to be over 4kg via ultrasound are not over 4kg when born (Relph et al. 2023). Therefore, lots of women are being incorrectly told that their baby is ‘big’. A US study found that one out of three women were told their baby was ‘too big’ based on ultrasound (Cheng et al. 2015). In this study the average birth weight of the group of babies suspected of being big was 7lb 13oz – ie. not big at all. Being diagnosed with a ‘big baby’ impacts on women’s experiences of their pregnancy. A study by Baddington et al. 2023 explored women’s experiences of ‘navigating an ultrasound prediction of a large baby in pregnancy’ and concluded that: ‘The prediction of a ‘large’ baby in pregnancy has undeniably negative impacts on women. We encourage midwives to scrutinise the dominant discourses of authoritative scans and problematic large babies, becoming vectors for critical thinking and resistance’.
Another US study concluded that ‘fetal biometrics [ultrasound assessment of size] had limited ability to predict SD [shoulder dystocia] and lack clinical usefulness’ (Newman et. al. 2022). So, predicting a large baby is inaccurate, does not improve outcomes and instead negatively impacts on women’s experiences.
Giving birth to a big baby is associated with an increased chance of particular outcomes – notice I am using the term ‘associated’ not ’caused’. The main complication associated with big babies is shoulder dystocia. The incidence of shoulder dystocia increases with the size of the baby. For example, it occurs with around 1% of babies weighing less than 3.9kg (8lbs 8oz), compared to 5–9% of babies weighing between 3.9kg and 4.5kg (9lb 9oz) (Politi et al. 2010). Other, less likely complications associated with big babies are severe perineal tearing (0.6%) and postpartum haemorrhage (1.7%) (Weismann-Brenner et al. 2012). I have previously written about how to reduce the chance of these complications:
- Birthing big babies and minimising the chance of shoulder dystocia: shoulder dystocia: the real story and gestational diabetes: beyond the labour
- Reducing the chance of tearing: perineal protectors
- Reducing the chance of postpartum haemorrhage: an actively managed placental birth might be the best option for most women
Care provider fear
However, research suggests that the complications associated with big babies may be due to interventions carried out when a baby is suspected to be big. Care providers are more likely to diagnose slow progress during labour and recommend a caesarean if they suspect the baby is big (Blackwell et al. 2009). Women who are told that they have a ‘big baby’, and are counselled about potential complications, are significantly more likely to choose a planned caesarean (Peleg et al. 2015). One study compared the outcomes of a group of women with suspected big babies with a group of women who unexpectedly gave birth to a big baby (Sedah-Mestechkin et al. 2008). Women who were suspected of having a big baby were three times more likely to have an induction or caesarean, and were four times more likely to have complications such as severe perineal tearing and postpartum haemorrhage. In this study there were no differences in the incidence of shoulder dystocia between the two groups.
Therefore, when a baby is suspected of being big, a woman has an increased chance of interventions during birth, and of experiencing complications caused by those interventions, even if the baby is not actually big.
The perception of a baby’s size influences outcomes more than the actual size of the baby
A Cochrane Review comparing induction of labour before 40 weeks for a suspected big baby with waiting for spontaneous labour, found that induction decreased the incidence of shoulder dystocia from 68 in 1000 to 41 in 1000. However, the review also found an increased rate of perineal tearing in the induction group of 26 in 1000 compared to 7 in 1000 in the spontaneous labour group. The review also notes that “antenatal estimates of fetal weight are often inaccurate so many women may be worried unnecessarily, and many inductions may not be needed.” There are a number of risks associated with inducing labour (see this post and this post) that need to be considered against the potential risks of of birthing a big baby.
NICE guidelines and World Health Organisation guidelines both state that induction of labour should not be carried out simply because a baby is suspected of being big. Interestingly, Queensland Health induction guidelines do recommend induction if a baby is estimated to be big via ultrasound. Yet the Queensland Health gestational diabetes guidelines state that “estimation of fetal weight by clinical assessment or USS can have significant margins of error”.
The estimation of a baby’s size via ultrasound is inaccurate. The complications associated with big babies may reflect care provider fear and practice. International guidelines do not recommend induction for a suspected big baby. Women need to be given this information before agreeing to an ultrasound scan aimed at estimating the size of their baby. Once the ‘big baby’ label is applied it cannot be removed and may alter the birth experience and outcome.