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 (GDM) - 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 GDM babies with non-GDM 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 [1]. Measuring the baby's abdomen with an ultrasound only predicts the weight of the baby within 15% of their actual weight [2]. Around half of the babies estimated to be over 4kg via ultrasound are not over 4kg when born [3]. Therefore, lots of women are being incorrectly told that their baby is 'big'. A US study found that...

Continue reading this article on Dr. Rachel Reed's website here.