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Is a 'big' baby a big problem?

Updated: 5 days ago

Do you need to have an induction if you have been told your baby is measuring 'big'? What does the research say?

At my last group class, 3 of the 5 women were being sent for Growth Scans due to measuring 'big' - that is 60%! When this happens, the word 'induction' starts being thrown about. So what does this mean for you, and what are your options?

Reading your GROW Chart

Inductions are commonly offered because baby is suspected to be measuring very large but is this necessarily an issue? To understand this we first need to understand how growth is measured. Baby's growth is tracked using two methods, fundal height measurements using a tape measure, and ultrasound scans. These measurements are then plotted on graphs to track baby's growth against "centiles". Centiles are lines on the chart, showing where in comparison to 100 babies yours compares. Take a look at this growth chart:

If you have been sent for Growth Scans, you will be able to access your Grow Chart on the Badger Notes App. Your chart is unique to you, and is created taking your height, weight and BMI into account (despite BMI not being a accurate measure of health - but more on that another time!)

Inconsistent vs Consistent Growth

When considering baby's size and growth, one thing to consider is consistency. If your baby has always measured on 90th centile consistently, then it purely means baby is big and that's not necessarily an issue. If your baby started off just over the 10th centile and gradually climbed up the centiles to the 90th, that's an indication that potentially something is affecting growth such as unmanaged gestational diabetes, and baby is growing faster and bigger than they would normally, and that may be a cause for concern.

Generally we grow babies appropriate to our size and a consistently small or large baby just means they're small or big! If you're 6'1" and your partner is 6'2", you're likely to have a baby up nearer the higher end of the centiles, and that's not surprising! It's genetics! And it doesn't mean you won't be able to birth your big baby vaginally without issue either.

Another thing to be aware of is comparing scan measurements against fundal height measurements. These are not like for like and shouldn't be compared against each other. If you agree to extra scans, be aware that particularly in the third trimester these can be less accurate and a small or large baby may be incorrectly picked up because scans can be 15-20% out.

"Antenatal estimates of fetal weight are often inaccurate so many women may be worried unnecessarily, and many inductions may not be needed." Induction of labour at or near the end of pregnancy for babies suspected of being very large (macrosomia) - Boulvain M et al 2016.

So for a baby estimated to weigh 4kg (the cut-off point usually used to define a 'big' baby), a 15% margin either side means the range of the estimate is from 3400g (7lbs 5oz) to 4600g (10lbs 4oz).

What about shoulder dystocia?

Induction is often suggested to reduce the chances of shoulder dystocia. This is when after a baby’s head has been born, one of the baby’s shoulders becomes stuck behind the woman’s pubic bone, delaying the birth of the baby’s body.

This happens in about one in 150-200 births, depending on what data you look at. We think it might occur less often when women are able to move about freely, so that’s why the estimates that come from of hospital-based studies and obstetric researchers tend to suggest that it’s a bit more common than when you look at data from home and birth centre settings. But either way, it’s not an everyday occurrence, though it’s not a rare event either.

Being able to move freely is a cruical sentence here - if you are offered an induction it is likely you will be asked to be connected to a CTG monitor which can restrict your movement, therfore increasing the risk of shoulder dystocia on top of having a big baby.

Along with this, Dr Sara Wickham makes these Key Points in this article:

  1. The vast majority of big babies (in fact 94% of those who weigh 4kg or more) won’t have shoulder dystocia.

  2. Shoulder dystocia doesn’t only occur in big babies, it can occur in small babies too.

  3. We can’t accurately