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Big babies, small babies, induction and grow charts - what’s the deal?

Have you been told that your baby is measuring big and they recommend inducing early? Or perhaps they suspect baby is small and needs to be birthed early? It can be confusing understanding how they predict the size of baby and what it means for your birth preferences! Is a big baby a genuine risk? Are there alternatives to induction? Read on….!

Suspected large or small baby? How do we know?

Inductions are commonly offered because baby is suspected to be measuring very large or very small, 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 this growth chart.

This baby is plotting just above the 90th centile, which means out of 100 babies on average, 90 would be smaller than this baby and 10 would be bigger, meaning this baby is larger than most. But is that a reason for induction (as this person has been recommended)?

Consistent vs inconsistent 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.

Similarly, a suspected small baby that’s consistently tracked along the same centile may just be naturally small, and not a cause for concern. But if their growth is dropping down the centiles it may indicate issues with baby’s growth and may be a reason for an earlier birth.

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. Similarly, if you’re 5’1” and your partner is also smaller, you’re likely to have babies on the lower end of the charts and that isn’t necessary an indication there’s something wrong with baby and their growth.

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.

Take this chart below. 4 digit measurements are scans, 2 digit are fundal height. This person has been told baby is “small” and has had extra scans. But baby is tracking on the 10th centile (which isn’t small!) and the 37 fundal height measurement is way off and indicating baby has jumped up to over the 50th centile, which is most probably an error.

Induction for small or large babies

Although a policy of very early induction (37 to 38 weeks) can prevent some shoulder dystocia cases from occurring, researchers have not been able to show that induction decreases the risk of brachial plexus palsy, and very early induction may carry other risks to the birthing person or baby. A policy of elective Cesareans for big babies likely does more harm than good for most birthing people: It would take nearly 3,700 elective Cesareans to prevent one permanent case of brachial plexus palsy in babies who are suspected of weighing more than 9 lbs., 15 oz.
Evidence on: Induction or Cesarean for a Big Baby -Rebecca Dekker, PhD, RN and Anna Bertone, MPH - Evidence Based Birth

The NICE guidelines state that a suspected large baby alone with no other medical indications ISN’T a reason for induction. And when inducing for a suspected small baby it’s important to understand the risk of unplanned caesarean, which is increased due to the fact that smaller babies do not cope well with the induction process. You also have the option to decline induction all together, or opt for an elective caesarean.

Half of the babies who were thought to be small – and whose mothers were told they were small, and whose mothers had extra screening and sometimes intervention (including induction of labour) because they were thought to be small – turned out not to have been small at all. So of women who were told that their baby was too small, only half of them actually had a baby that was small. There was just as much chance that the assessment was wrong as that it was right.
Trying to identify small babies during pregnancy may do more harm than good… By Dr Sara Wickham

If you’d like to read more on the topic, I’d recommend the following articles:

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