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Big babies, fundal height measurement and third trimester growth scans

Big babies, fundal height measurements, third trimester growth scans - what’s all the fuss about?


HELP! My baby is too big. Or maybe too small? Who really knows?


Well the answer is no one, no one really knows the size of your baby until they’re born and we can actually weigh them. But it seems that not a week goes by that I don't hear from a client, friend or follower who has been recommended a growth scan because their baby is measuring "too big" *eye roll*.


So what do we mean by too big? An LGA baby - large for gestational age, is any baby who weighs more than the 90th centile, also referred to as fetal macrosomia. Centiles are a way of tracking your baby's size against the national average, at any midwife appointments you attend in the third trimester your fundal height measurements (the distance in centimeters from the pubic bone to the top of the uterus) will be taken and plotted on a graph showing these centile lines, should you consent of course. So for example, if your baby is on the 50th centile then they are measuring the same as the median average for their gestation. Anything above is larger, anything below is smaller. Here's a breakdown of fundal height and centile plotting. If you're having these measurements taken you will have a copy of this graph, also known as a GROW chart, or similar, in your notes…




So at these appointments, if your fundal height is measuring above the 90th centile it will likely be suggested that you need a scan to check if baby is actually that big, or a growth scan may be requested if you are consistently measuring within the same centile range until one appointment where there is a lot of growth we wouldn't expect. For example jumping from always being around the 50th to then measuring around the 80th centile.


Is fundal height an accurate mode of measurement?

Not really, or at least not for estimating your babies size. It doesn't actually tell us much at all about the size of your baby - just your bump, because there's not just a baby in there, there's also water and placenta, and some other muscle, organs and body parts behind and underneath too! You could have a mighty large bump, but just a heavy load of water, or you could have a teeny tiny bump hiding a mighty large babe (my first tiny bump somehow housed an 8lb10oz baby!). Fundal height measurement plotting can show us how your baby is growing, as demonstrated above if there is a large jump up or down centiles but it can also be influenced by things like your baby's position in the uterus or who takes the measurement - different health care professionals will likely measure from different points and we also have to allow for a margin of human error. A 2016 study entitled "Diagnostic accuracy of fundal height and handheld ultrasound-measured abdominal circumference to screen for fetal growth abnormalities" which looked at comparing fundal height measurement against ultrasound scanning for predicting fetal growth restriction and fetal macrosomia states "Numerous studies have shown that fundal height (FH) has a poor positive predictive value for identifying abnormally grown fetuses"[1]


Moving on, let's say that your fundal height measurements trigger a scan being booked, this won't always be presented as a choice. You may be told you "need to have a scan", but this is your decision to make and you can absolutely decline if you do not feel it is necessary. There are benefits and risks to accepting a late stage ultrasound, but in an otherwise healthy and "normal" pregnancy, there is very little evidence to suggest that a third trimester ultrasound scan would be able to accurately diagnose an LGA baby, and even less evidence to suggest that these scans can prevent adverse outcomes. Third trimester scans can be useful however for checking amniotic fluid levels, checking placental blood flow and function, they can also be helpful to check what position your baby is in if your health care provider isn't sure and is concerned they may be in a transverse or otherwise non-optimal lie. In many trusts, third trimester growth scans are also offered to pregnant people for other reasons too such as those with a BMI over 35, people with gestational diabetes, or those on other “high risk” pathways.


What happens at the scan?

These scans are not just a nice chance to see your baby again and you won’t be able to buy photographs, however the sonographer will work much in the same way as at your 12 and 20 week scans, focusing on getting as accurate measurements of your baby as they can - incredibly difficult due to babies position in the uterus in the third trimester and how much room they now take up in there. They are taking measurements of your baby’s head, abdomen and femur length (the bone of the thigh) which generate an estimated weight of your baby at that time. They may show you the screen and talk through what they are doing or they may not, it just depends on who you see on the day. When the scan is complete, the sonographer will plot your baby’s estimated weight on to your GROW chart and check to see if it matches up with your fundal height line, you will then usually be asked to head back to the waiting room while the scan is reviewed by a Dr/Consultant or if everything looks OK, or they cannot check it immediately, they may just offer a follow up telephone call. This process can differ slightly across trusts so if you are unsure of the process, be sure to double check with the midwife referring you for the scan beforehand.


On the rare occasion that there are any immediate concerns, for example extremely restricted growth, abnormal fetal development, placental function problems or extreme polyhydramnios (too much amniotic fluid) then you may be asked to attend your trusts maternity assessment unit or antenatal day unit for further investigation straight away.


How accurate are the scans?

"Unfortunately, an accurate diagnosis of LGA or macrosomia can be made only after weighing the infant at birth because clinical estimates and ultrasonography have proven to be unreliable"[1]

It has long been known and accepted that third trimester scans performed after the 32nd week of pregnancy can be up to 15% inaccurate, this means that a baby estimated to be 8lb on a scan could actually be as big as 9lb2 or as small as 6lb8oz. As discussed previously in this blog post, a baby weighing over 9lb is classed as LGA (over the 90th centile), but we could be diagnosing babies who only weigh 8lbs as having fetal macrosomia, we’ll discuss the outcomes of this label in a moment. A 2018 study also found that lack of experience and insufficient training and audit in the population of sonographers undertaking these scans can cause further inaccuracies.[2]


A recent study found that “Only 81% of the scans done between 34 and 37 weeks gestation fell within the accepted 15% margin of error between estimated fetal weight and actual weight compared”[3] which is incredibly shocking as it means that not only were 81% of people in this study told an estimated fetal growth rate which was up to 15% inaccurate, but that 19% of people in this study were told an estimated fetal growth rate which was MORE than 15% inaccurate. Furthermore a 2018 systemic review entitled “The accuracy of ultrasound estimation of fetal weight in comparison to birth weight” found that the mean percentage error varied from -6.88% inaccurate to +22.16% inaccurate across a broad spectrum of studies.[4] To put this in to context, if your baby’s actual weight was 7lb (which is the average weight for a full-term baby) a scan could estimate you as housing a 9lb baby, this is not only a difference of 2 whole lbs, but would also mean that you would be classed as having a large for gestational age baby, above the 90th centile which comes with a whole heap of repercussions, when your baby was actually bang on average weight for their gestation.


So let’s talk about those repercussions, what’s the big deal around big babies anyway?

If health care providers think you are having a big baby they will likely want to schedule an induction, maybe not straight away but usually before your baby would be ready to come if left the heck alone, of course you can decline this but let’s discuss their reasoning first. Their reasoning for wanting to induce is that having a large baby can increase the risk of shoulder dystocia, postpartum hemorrhage(PPH) and third degree tear, but absolutely paradoxically - having an induction also increases the risk of shoulder dystocia, postpartum hemorrhage and shoulder dystocia - make it make sense. They’re literally saying, we can’t let you keep this big baby in any longer we need to get them out ASAP in case you have a shoulder dystocia, PPH or bad tear so let’s do something which increases even further your chances of having a shoulder dystocia, PPH or third degree tear, but they’re very rarely actually explaining to you the risks of the induction and instead scaremongering you in to thinking you cannot birth a big baby.


Furthermore, when they say the risk of these things increases with a large baby, this is true, but the risk increases such a small amount that for most it’s not even a concern, but how can you know this is you don't know that actual statistics. For example, the data is hard to find accurately around PPH because it really does vary from trust to trust and can be down to many different contributing factors or can seemingly be completely random. But in 2012-13 in the UK the rate of PPH occuring was in 13.8% of births[5], a study on macrosomia in Tanzania of 4000+ deliveries found that the rate or PPH in people giving birth to LGA babies was 17.5%[6]. Professor of Midwifery at the University of Western Sydney and Australian College of Midwives’, Hannah Dahlen, was quoted in The Age as saying the induction rate “is the biggest predictor of having a haemorrhage”.[7] with studies finding rates of PPH in up to 24% of those who are induced.[8] What we do know is that the rate of PPH is rising year on year here in the UK, in line with inductions which are rising rapidly year on year, it would seem that these things are likely intertwined and not mere coincidence. Now if we look at the risk of having a third degree tear, the risk of having a third degree tear in first time parents is 6%, the risk of a third degree tear with a large for gestational age baby is 7.4%[9] an incredibly small increase and still over 90% chance of not having one of these tears. Unfortunately at this time I cannot find any studies which accurate demonstrate the percentage risk of having a third degree tear when labour is induced but many which state that it categorically does, such as this one which looked at 33,000+ deliveries in 2021 and concluded that “The induction of labor is associated with an increased risk of severe tearing.”[10] I won’t discuss shoulder dystocia statistics here as I am currently working on a blog post and podcast episode about shoulder dystocia as a standalone topic but if you wanted to look it up you would see that induction of labour again increases the risk alongside LGA babies.


So really, we should be taking the recommendation of induction with a pinch of salt. I’m not at all saying don’t accept - that is a decision only YOU can make. I’m also not saying that any of these things are “too” risky, or not risky enough, because again, that is not for me to say! What’s too risky for one person, will be completely different to another person and that is fine. Maybe something going from a 1% risk to a 2% risk is enough for you to accept an induction, likewise something having an 80% risk of an adverse outcomes might still not be high enough for you to change your plans - that’s good, it means we are all using critical thinking AND tuning in to our intuition right, that’s why we’re all coming to different decisions and that’s really important to highlight because that’s how maternity care should be. You should be treated as an individual who can make your own choices based on what feels right for you, not based on blanket polices with little evidence behind them. Did you know that 82% of the Royal College of Obstetricians and Gynaecologists guidelines are not based on grade A quality evidence and research and that 40% of guidelines are not evidence based AT ALL, things that are being suggested to you are not always in your best interests. And i’m not anti medical care, anti induction, anti midwife etc etc at all, I think these things can be incredibly important and life saving in some instances, I’m just saying you are an individual, your pregnancy is yours, please don’t be fear mongered in to making decisions you’re not comfortable with based on a scan which may not be accurate.


Before I round off this blog post I want to mention one more thing, you can birth a big baby. You absolutely can, maybe the scan *is* accurate, or maybe you decline the scan but your baby happens to be big, maybe nobody has a clue that you’re growing a giant baby so it’s never even an obstacle that comes up for you. If your baby is big you can absolutely still have a physiological vaginal birth with no outside interference, no adverse outcomes and no assisted mode of delivery. People give birth to big babies every single day, most have no idea their babe was even going to be classed as large. It’s just a variation of normal, there will always be small babies and there will always be big babies. Your baby is the perfect size for your body and your baby knows how to be born, your body knows how to birth your baby. Nobody can ever guarantee you a risk free pregnancy or birth, that’s because nothing in life is risk free - stepping out of bed comes with risks, it’s just a normal part of life. But ultimately birth is safe and you can do this, don’t let anyone convince you that you can’t! There is SO much more I could say on this topic but it would end up being 900 pages long so I’m going to stop here, for now.


I hope this was a helpful read and has given you some food for thought, if you’d like to discuss this or any other aspects of your pregnancy and birth preparation further then don’t hesitate to book in for a power hour with me, link here: https://calendly.com/thedungareedoula/power-hour?month=2023-05


Here is the reference section if you would like the actual studies for future use or further reference, but do be aware that as with most research around pregnancy and birth a lot of the studies are also talking about stillbirth and or maternal mortality so only read if you can handle having that in your mental space right now.




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