Episode 182 Big Baby Update

Remember my popular Episode 29 on big babies from five years ago? Today I'm diving into what's changed since then, spurred by Warwick University's Big Baby trial, new research examining whether early induction truly prevents complications in pregnancies where scans predict larger babies.

The most startling revelation? A whopping 60% of babies predicted to be above the 90th centile weren't actually that large at birth. This  margin of error should give us serious pause when recommending interventions based on third-trimester scans. 
What also troubled me was the study design itself. The "standard care" group still saw about 75% of women receiving some form of intervention rather than experiencing spontaneous labour. With only six days average difference in delivery timing between groups, is there really enough distinction to draw meaningful conclusions? The recorded times between head and body delivery (just over one minute in both groups) further suggests we might be rushing physiological processes unnecessarily.

For pregnant women hearing "your baby might be big," take these predictions with healthy skepticism. Having a larger baby doesn't automatically mean a difficult birth lies ahead. For healthcare providers, perhaps it's time we questioned whether we've trapped ourselves in a cycle of intervention based on imprecise predictions and exaggerated risks.

Listen to this update alongside my original Episode 29 Big baby and episode 102 Shoulder Dystocia. Subscribe, rate, and share if you found this valuable!

https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(25)00162-X/fulltext

Thank you all for listening, My name is Florence Wilcock I am an NHS doctor working as an obstetrician, specialising in the care of both mother and baby during pregnancy and birth. If you have enjoyed my podcast please do continue to subscribe, rate, review and recommend my podcast on your podcast provider.
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Chapters

• 0:00 Welcome to the OBSpod
• 0:58 Revisiting the Big Baby Issue
• 2:18 The Big Baby Trial Explained
• 5:32 Standard Care vs. Intervention Results
• 10:22 Were These Babies Actually Big?
• 16:04 Critical Analysis and Practical Advice
• 22:00 Episode Closing and Resources

Transcript

Florence: 0:00

Hello, my name's Florence. Welcome to the OBSpod. I'm an NHS obstetrician hoping to share some thoughts and experiences about my working life. Perhaps you enjoy Call the Midwife. Maybe birth fascinates you, or you're simply curious about what exactly an obstetrician is. You might be pregnant and preparing for birth. Perhaps you work in maternity and want to know what makes your obstetric colleagues tick, or you want some fresh ideas and inspiration. Whichever of these is the case and, for that matter, anyone else that's interested, the OBS pod is for you. Episode 182 Big Baby Update.

Florence: 0:58

One of my most popular episodes of all time was episode 29, big Baby. I can't help but think that this is because it's such a common problem that women encounter during their pregnancy or that midwives encounter when they're delivering antenatal care. I'm not going to repeat everything that I said in episode 29. So if you haven't listened to episode 29, go back and take a listen, because that's a really good place to start. Although I recorded it nearly five years ago, much of what I had to say then is still absolutely relevant. But much of what I had to say then is still absolutely relevant. So if I'm telling you to go back and listen to that episode, why am I recording a new one. What has changed? Well, the thing that's prompting me to record today is the publication of the Big Baby Trial. This was a trial undertaken by Warwick University and its results were eagerly anticipated by many of us in the maternity world.

Florence: 2:23

This issue, as I described in episode 29 what to do when a woman came to see us with a scan that suggested the baby might be big, the baby might be in the top 10 percent of the population, or 90th centile or above and the reason we have concern that we should be doing anything is because of our worry about complications at birth, in particular shoulder dystocia, which I covered in episode 102. That's where the baby's shoulder gets wedged behind the woman's pubic bone after the birth of the head. This can lead to complications for the baby and it can lead to more severe tears for the mum. So definitely a good thing to avoid and definitely has an association with bigger babies, although that isn't the only factor that influences whether you may or may not have a shoulder dystocia. At this point, I feel it's fair to emphasise that many women and babies that will have a shoulder dystocia will not actually have any complications at all.

Florence: 3:55

Shoulder dystocia is defined as needing to do additional manoeuvres to release the baby's shoulder. To release the baby's shoulder, normally during a vaginal birth, straightforward one the baby's head is born by extension and then the baby rotates, something we call restitution and external rotation. So that's the baby's body coming down the birth canal and under the pubic bone, and there's often a minute or two between the birth of the head and that occurring. And patiently, and not try and help the baby be born until those things have happened, you need to wait for that normal mechanism, and sometimes that means waiting for a whole other contraction which can feel like a really long time and can be quite disconcerting. As a junior member of the maternity team, shoulder dystocia is defined as needing to do additional things. So rather than just being able to place our hands on the baby's head and help with the birth of that baby, if needed, we need to do some additional manoeuvres, and you can go and listen to episode 102 to find out a bit more about what those manoeuvres are. So let's turn to the big baby trial. So the big baby trial was looking at if your baby was estimated as being big on an ultrasound scan, that is, over the 90th centile, so in the top 10% of where we'd expect baby's growth to be and the heaviest 10%. If we offered an intervention, that is, an early induction of labour, so starting the labour off artificially before the woman is ready to go into spontaneous labour, whether or not that would make a difference to the chance of shoulder dystocia happening. The idea is that you cut short the baby's ability to grow even bigger, so the baby has a slightly lower birth weight because it's born a little bit earlier and therefore you might avoid the possible complication of shoulder dystocia.

Florence: 6:34

So that was the plan. They aimed to look at around 4,000 women, so they were going to compare two thousand in one arm. So the intervention arm. So that is offering an earlier induction, and in this case they were talking about induction between 38 weeks and 38 and four. So 38 weeks, four days. They were going to compare this with standardised care. So if you were having standard care, that just meant you were having whatever the midwives and obstetricians were normally doing. Well, certainly in my unit there's quite a lot of variation about what people are doing. Do we offer cesarean? Do we offer induction? If we offer induction or cesarean, at what point? How many weeks? So standard care, yes, that is standard for what that hospital were doing. So that's a kind of control arm. But standard care suggests everyone's having the same care and when I come on to look at the results that really is not the case. And in the study they were assuming a rate of shoulder dystocia of around 4%. Around 4% or 1 in 25 births in that cohort of bigger babies. So that's the plan.

Florence: 8:23

Let's turn and look at what actually happened and I'm not going to go into every minute detail. I'm going to put a link to the study in the show notes. The first thing that I noticed in the write-up is that they stopped the trial early, and the reason for stopping the trial early is that the levels of shoulder dystocia were lower than they'd anticipated. So they didn't think they were going to be able to pick up a difference in levels of shoulder dystocia between the investigation arm that's, the earlier induction and the standard care arm. So straight away, the issues were that the complication of shoulder dystocia in those estimated bigger babies was lower than expected. In those estimated bigger babies was lower than expected.

Florence: 9:26

The next thing that you need to look at is the timing of birth. So they were aiming for a difference of timing of birth of 10 and a half days on average between the two groups, and they hoped that this would result in a birth weight difference of around 300 grams. When you look at the results, they ended up with only an average of six days difference. So much closer differences in gestation, at birth and only 163 grams difference in birth weight. And I think this is partly because of this issue about standard care that I mentioned earlier. When you look at those having standard care, only around a quarter of women actually went into, so around 70% of women ended up with some form of induction or cesarean anyway as part of standard care. Around a quarter were induced between 38 and 5 and 39 weeks, so a couple of days later than the induced group. Around a third were induced between 39 plus one and 39 plus six weeks.

Florence: 11:08

And to me, what I really want to know is not about whether I should be suggesting induction at 38 to 38 plus 4 or 39 to 39 plus 4. I want to know should I be recommending induction at all? Should I be intervening at all? What would happen if these women went into spontaneous labour, because we know some of them will go at 38, 39, and some of them will go at 40 or 41. So I think one of the major problems with the trial for me is standard care. What is standard care? And standard care is in itself an intervention, it would seem. An intervention. It would seem because only 261 women in the standard care arm went into spontaneous labour after 38 plus four, so only a quarter of women actually went into labour after that date when their body and their baby were actually ready. And that goes back to what I talked about in episode 29. How many hundreds or possibly thousands of women and babies need to have an induction to prevent potentially one case of shoulder dystocia?

Florence: 12:42

One of the other things that interests me about this trial was how you define shoulder dystocia, and shoulder dystocia is a clinical diagnosis. Did the care providers need to do some additional manoeuvres to help with the birth of the baby and the baby's shoulders? Now, at work, we have a very clear pro forma to fill up with. What manoeuvres and interventions are we doing to release the baby's shoulders and how long was the delay from the birth of the head to the birth of the baby's shoulders and body? So I had a look at the results, looking at the time between the delivery of the head, the birth of the head and the birth of the body in minutes. And I found this really quite astonishing Because the mean time between the birth of the head and the birth of the body in minutes for the induction, the intervention arm, is 1.09 minutes and with standard care was 1.21 minutes. Now I don't want to be too negative, but so to me these times are unbelievably quick and I appreciate that that's because there were very, very few cases of shoulder dystocia.

Florence: 14:34

But a minute between the birth of the head and the birth of the shoulders in both arms is incredibly fast and doesn't suggest that we're waiting for the normal physiological mechanisms of birth. Even If you wait for another contraction, it's usually a good maybe two, sometimes even three minutes before the birth of the rest of the baby, the birth of the rest of the baby. And whilst this might seem unimportant because they had very few shoulder dystocias in the study, it is important because if you take your hands around the baby's head and perform traction, which is the routine, next step, potentially with a vaginal birth, if you do that too early, before the baby has come into the right position, restituted, the head is restituted and external rotation has occurred, then you could mislabel and call something a shoulder dystocia. That wasn't so, luckily, because there were so few shoulders of dystocias. We don't have to think too much about what we're defining as a shoulder dystocia, but it it calls into question for me some of the midwifery and obstetric practice, and this was a large multi-centre trial in the UK, so don't get me wrong, I'm not criticising any particular unit. I'm just saying maybe our skills at assisting a physiological birth and our education around the normal mechanism of birth needs a little bit of freshening up.

Florence: 16:51

Ok, now for the really big question Were these babies actually big? Were these babies actually above the 90th centile? In episode 29 I talked quite a lot about the margin of error with ultrasound scan in the third trimester, in the last third of pregnancy. So what did the big baby trial find? Well, for me this is the single most important finding of the trial. So remember we said that these were babies that were on the top 90th centile, approximately 60%. In both groups. So the induction group, the intervention and the standard group the babies were not above the 90th centile. They weren't as big as we thought they were. So 40% in 40% of cases. Yes. So if you're told you've got a big baby above the 90th centile. There's a 40% 4 in 10 chance that that is correct and a 60% or 6 in 10 chance that the baby is smaller and that's massive. Because that means we're doing all this intervention to prevent a complication that not only is rare in those bigger babies, but also your baby might not be big at all in the first place.

Florence: 18:48

And whether we're doing the intervention described in the big baby trial of induction between 38 weeks and 38 weeks and four days, or whether we're doing standard care, which appears to be a lot of induction before 40 weeks, why aren't we just leaving things alone? I can't help but feel we've kind of tied ourselves in knots. We don't know what to advise. To me, the big baby trial hasn't really helped me give some really good information quality information to women about their chances, quality information to women about their chances, and we're doing an awful lot of intervention. And don't get me wrong if your baby has a bad shoulder dystocia and has a complication as a result, that is awful and if you have a bad tear as a result, that is equally can be very difficult and prolonged recovery with consequences for you. But the chance of that happening is really small and yet we're building this fear fear about the size of a bigger baby and we're doing a lot of intervention just in case and not thinking about the potential complications of that intervention. Further down the line, and to be fair to the big baby trial, they did look at things like maternal postpartum hemorrhage and bad tears, so they did try and look at this, but that's still very short-term outcome of a few months. They're not looking at what was the outcome longer term for these mothers and babies beyond the first few months of motherhood and of life.

Florence: 21:04

Finally, before I leave this topic, I'm going to talk about the results of intention to treat and per protocol, because this is a slightly weird way they decided to analyse the results. So intention to treat is we intended to do this to this group. This group we intended to have standardised care and this group we intended to have induction and that's how we're going to analyse the results and in that way, intention to treat, there wasn't a difference in shoulder dystocia between the two groups. Then they looked at what they called per protocol outcomes, which meant that anyone that had had a baby before 38 plus 4 induction or going into spontaneous labour. They looked at those babies and those mothers in comparison with the others. And for that they said there was a small difference in shoulder dystocia between the two groups.

Florence: 22:25

But to me it's kind of moving the goalposts Because when I'm talking to a woman in clinic I can only advise her on what maybe I'm recommending in terms of treatment. I can't in advance know when she might or might not go into spontaneous labour and say to her well, if you go into spontaneous labour or you're induced for another reason before that, then it will make a difference. But if I intend to induce you and you agree to that induction, it won't make a difference. I mean, how useful is that? Maybe it's because I'm not an academic and scientist, I'm a jobbing obstetrician, but to me that's really unhelpful. And when they put out the big baby trial, that's why you will see in some of the headlines and some of the reports the fact that they suggested that this reduced the chance of shoulder dystocia. So I'm going to put a little health warning on that. Per protocol analysis rather than intention to treat analysis. To me with my kind of common sense, jobbing brain, it kind of doesn't make sense.

Florence: 23:49

Okay, now for the zesty bit. If you're a pregnant woman. My zesty bit would be take any third trimester or late pregnancy growth scan result with a pinch of salt In this study. The scans were done for other reasons, so the scans just by coincidence found that the baby was big, and you might end up in that situation too. But bear in mind that figure, that statistic, that actually six out of ten babies that were thought to be the 90th centile weren't. And also, having a baby on the 90th centile, don't think that that means you're automatically going to have a scary and difficult birth. Yes, you may have discussions with the obstetrician and midwife, you may have choices to make, but try and hold that thought and ground yourself with that thought.

Florence: 25:08

If you're working in maternity, then I think we need to also take the whole big baby study with a pinch of salt. Has it added to our knowledge? I'm not really sure it has, and that's a shame. Why is it that the research we're trying to do on this occasion has really not helped push things forward? Or, to look at it a different way, maybe it has. Maybe it's telling us that we should not be inducing and intervening and worrying about the size of a baby when the baby's on the bigger side. Maybe that is what the conclusion is, but it's difficult when perhaps the hypothesis, the theory we had in mind, is not proved by the research we then undertake. So I hope that's been helpful. Those are my thoughts about the big baby trial, and do go back and listen to episode 29, my original big baby episode, and in particular the wonderful poem at the end which gives you the viewpoint from Jenny Jenny Thirlwell, my good friend Jill's daughter, about what it was like in her pregnancy, with this yo-yoing between too big, too small, just right, how it feels to have endless scans giving different answers.

Florence: 27:04

I very much hope you found this episode of the OBS pod interesting. If you have, it'd be fantastic if you could subscribe, rate and review on whatever platform you find your podcasts and review on whatever platform you find your podcasts. As well as recommending the OBS pod to anyone you think might find it interesting, there's also tons of episodes to explore in my back catalogue from clinical topics, my career and journey as an obstetrician and life in the NHS more generally. I'd like to assure women I care for that I take confidentiality very seriously and take great care not to use any patient identifiable information unless I have expressly asked the permission of the person involved on that rare occasion when it's been absolutely necessary. If you found this episode interesting and want to explore the subject a little more deeply, don't forget to take a look at the programme notes, where I've attached some links.

Florence: 28:17

If you want to get in touch to suggest topics for future episodes, you can find me at theobspod, on twitter and instagram, and you can email me theobspod at gmailcom. Finally, it's very important to me to keep the ObbBS pod free and accessible to as many people as possible, but it does cost me a very small amount to keep it going and keep it live on the internet. So if you've enjoyed my episodes and, by chance, you do have a tiny bit to spare you do have a tiny bit to spare you can now contribute to keep the podcast going and keep it free via my link to buy me a coffee. Don't feel under any obligation, but if you'd like to contribute, you now can. Thank you.