Induction for big baby: what's the evidence?

Been offered induction for big baby?

Well, let’s start with a really important fact.

Researchers admit that, “it is not clear whether it is better for women with big babies to have their labour induced or to wait for labour to begin naturally.” (Warwick Clinical Trials Unit.)

In other words: they don’t know whether it’s better to induce labour or to wait.

In fact, that’s part of the rationale for the Big Baby Trial being carried out in the UK.

So that might be all you need to know. That we don’t know whether induction leads to better outcomes for suspected big babies or not.

But if you’d like to understand the issues a bit more, read on. This blog post is based on my chapter on this topic in In Your Own Time. It’s called Does my baby look big in this?, which I will confess is my favourite chapter title ever. It gives all the evidence and, I am delighted to tell you, has already helped loads of women and families to understand the evidence and find the confidence to make the decisions that are right for them.

What’s the problem?

But let’s get back to explaining the problem in this area. In a nutshell, the idea of inducing labour for suspected big babies is based on a series of assumptions about how we might be able to solve one problem.

Unfortunately, those assumptions don’t really stand up to scrutiny.

The problem we’re trying to solve is something called shoulder dystocia.

“[Shoulder dystocia] 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.” (Warwick Clinical Trials Unit.)

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.

Defining shoulder dystocia

Here’s a bit more on what the big baby trial researchers say about shoulder dystocia:

“[Shoulder dystocia] 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. Most babies born that have experienced shoulder dystocia will have no long term complications. But for some babies this can cause a stretching in the nerves of the neck, which may cause long-term weakness in the arm. We know that shoulder dystocia occurs more often in bigger babies but there is uncertainty in how often this actually occurs.” (Warwick Clinical Trials Unit.)

The medical term macrosomia is sometimes used here. It just means ‘big baby.’ You might also see ‘suspected macrosomia.’ That’s when someone thinks an unborn baby might be big at birth. As you’ll see in the next section, we’re not very good at guessing that, though.

The problem with the induction for big baby logic

In Your Own Time was written to help parents and professionals better understand the issues and the evidence relating to the current induction epidemic. Looks at the evidence relating to due dates, ‘post-term’, older and larger women, suspected big babies, maternal race and more.

I don’t disagree with what the researchers say in the statement above. But I don’t think it tells the whole story, and there are a few things that I would like to add. (If you’d like more depth on this and to see the evidence, please see In Your Own Time.)

The vast majority of big babies (in fact 94% of those who weigh 4kg or more) won’t have shoulder dystocia.
Shoulder dystocia doesn’t only occur in big babies.
We can’t accurately predict which babies will be big.
Only a few of the babies who have shoulder dystocia will have a serious problem anyway (and remember that only one in 150-200 babies have shoulder dystocia in the first place.)
Induction has risks and downsides which have to be weighed up against any possible benefits.
There may be other things we can do to prevent shoulder dystocia, like not having women laying on their backs to give birth.

The ultrasound problem

Today I want to focus on one of the key issues and something which is affecting many women today. That is, the idea that ultrasound weight guesses are a good predictor of whether your baby is likely to be big.

Spoiler alert: they aren’t.

As above, the idea of using ultrasound to estimate the weight of a baby is linked to a couple of other ideas that aren’t supported by evidence. We know that 94% of large babies won’t have a problem, and we as yet have no evidence that induction is beneficial or that it will reduce the chance of your baby having shoulder dystocia. Despite this, if your baby is deemed by ultrasound to be big, you’re likely to be offered an induction.

There are actually a good number of studies which have shown that ultrasound weight guesses have a wide margin of error. With babies, the studies show a 15% margin of error either way.

“So for a baby estimated to weigh 4kg (the cut-off point usually used to define suspected macrosomia), a 15% margin either side means the range of the estimate is from 3400g (7lbs 5oz) to 4600g (10lbs 4oz). That’s quite a range. And it’s still only an estimate, not a guarantee. A few babies will weigh more or less than a weight that falls within the 15% margin.” (Wickham 2021).

If you’d like more evidence of how ultrasound guesses are wrong, have a look at any of my Instagram posts about suspected big babies. The comments are absolutely full of people sharing the difference between the ultrasound estimate of their baby’s weight and their baby’s actual weight at birth.

More evidence

In 2022, a study published in the American Journal of Perinatology by Newman et al (2022) again showed that fetal biometrics (or the measurements taken during an ultrasound) “have limited ability to predict shoulder dystocia and lack clinical usefulness.”

In other words, this study again showed that ultrasound measurements cannot tell us which babies will have shoulder dystocia at birth.

The study looked at data from more than 1700 women who had uncomplicated births.

They looked at sociodemographic factors (things like age and ethnicity) and maternal anthropometrics (things like someone’s weight) and found that there were no differences between those whose babies had shoulder dystocia and those who did not.

The researchers could find no relationship between the measurements taken at ultrasound and a baby’s chance of experiencing shoulder dystocia.

In fact, only one thing was associated with a higher chance of shoulder dystocia, and that was whether or not the woman had an epidural. Those who had epidurals were more likely to experience shoulder dystocia.

This again begs the question of whether we are focusing in the wrong direction.

So why are we offering induction?

This study adds further weight (pun intended) to the question of why so many women are being offered induction for a suspected big baby. The prevention of shoulder dystocia is the main reason for offering this, as I explained above. And yet, as this study again shows, ultrasound estimates of a baby’s possible weight and size do not predict which babies will be larger and/or have shoulder dystocia.

I do have an answer to that question of why induction is being offered, by the way. It’s because what’s happening in the maternity services isn’t based on the evidence. It’s based on tradition, culture and some old ideas that we really need to move on from. More about that in In Your Own Time too.

If you find yourself being offered an ‘induction for big baby,’ make sure you’re informed. Ask questions. Do some reading. Look at the evidence for yourself, if that’s your thing. But please, be aware that the assumptions don’t always match up to the evidence. And make the decisions that are right for you.