Updated: June 2023
Big babies are normal in well resourced countries. Over 10% of babies born in the UK and Australia weigh 4kg (8lb 13oz) or more. Healthy well nourished women grow healthy well nourished babies. Genetic factors also influence the size of babies (big babies run in families); and each baby a woman has usually weighs more than the last. Babies also continue to grow at the end of pregnancy (because placentas continue to nourish them rather than switch off) – so a baby will be bigger at 42 weeks than they were at 40 weeks.
However, abnormal blood glucose levels (BGLs) – with uncontrolled gestational diabetes (GD) – can also cause a baby to grow big. Babies who are big because of high BGLs are a different shape to ‘normally’ large babies. In particular, their shoulders and chest are larger and fatter, and they are more likely to encounter complications at birth. Unfortunately, research into big babies usually combines the outcomes for GD babies with non-GD babies.
Estimating size
The only way to accurately assess the weight of a baby is to weigh them after birth. Clinical assessment ie. palpating and measuring pregnant bumps is incorrect more than 50% of the time (Chauhan et al. 2005). Measuring the baby’s abdomen with an ultrasound – only predicts the weight of the baby within 15% of their actual weight (Rossi et al. 2013). Around half of the babies estimated to be over 4kg via ultrasound are not over 4kg when born (Relph et al. 2023). Therefore, lots of women are being incorrectly told that their baby is ‘big’. A US study found that one out of three women were told their baby was ‘too big’ based on ultrasound (Cheng et al. 2015). In this study the average birth weight of the group of babies suspected of being big was 7lb 13oz – ie. not big at all. Being diagnosed with a ‘big baby’ impacts on women’s experiences of their pregnancy. A study by Baddington et al. 2023 explored women’s experiences of ‘navigating an ultrasound prediction of a large baby in pregnancy’ and concluded that: ‘The prediction of a ‘large’ baby in pregnancy has undeniably negative impacts on women. We encourage midwives to scrutinise the dominant discourses of authoritative scans and problematic large babies, becoming vectors for critical thinking and resistance’.
Another US study concluded that ‘fetal biometrics [ultrasound assessment of size] had limited ability to predict SD [shoulder dystocia] and lack clinical usefulness’ (Newman et. al. 2022). So, predicting a large baby is inaccurate, does not improve outcomes and instead negatively impacts on women’s experiences.
Birth complications?
Giving birth to a big baby is associated with an increased chance of particular outcomes – notice I am using the term ‘associated’ not ’caused’. The main complication associated with big babies is shoulder dystocia. The incidence of shoulder dystocia increases with the size of the baby. For example, it occurs with around 1% of babies weighing less than 3.9kg (8lbs 8oz), compared to 5–9% of babies weighing between 3.9kg and 4.5kg (9lb 9oz) (Politi et al. 2010). Other, less likely complications associated with big babies are severe perineal tearing (0.6%) and postpartum haemorrhage (1.7%) (Weismann-Brenner et al. 2012). I have previously written about how to reduce the chance of these complications:
- Birthing big babies and minimising the chance of shoulder dystocia: shoulder dystocia: the real story and gestational diabetes: beyond the labour
- Reducing the chance of tearing: perineal protectors
- Reducing the chance of postpartum haemorrhage: an actively managed placental birth might be the best option for most women
Care provider fear
However, research suggests that the complications associated with big babies may be due to interventions carried out when a baby is suspected to be big. Care providers are more likely to diagnose slow progress during labour and recommend a caesarean if they suspect the baby is big (Blackwell et al. 2009). Women who are told that they have a ‘big baby’, and are counselled about potential complications, are significantly more likely to choose a planned caesarean (Peleg et al. 2015). One study compared the outcomes of a group of women with suspected big babies with a group of women who unexpectedly gave birth to a big baby (Sedah-Mestechkin et al. 2008). Women who were suspected of having a big baby were three times more likely to have an induction or caesarean, and were four times more likely to have complications such as severe perineal tearing and postpartum haemorrhage. In this study there were no differences in the incidence of shoulder dystocia between the two groups.
Therefore, when a baby is suspected of being big, a woman has an increased chance of interventions during birth, and of experiencing complications caused by those interventions, even if the baby is not actually big.
The perception of a baby’s size influences outcomes more than the actual size of the baby
Induction?
A Cochrane Review comparing induction of labour before 40 weeks for a suspected big baby with waiting for spontaneous labour, found that induction decreased the incidence of shoulder dystocia from 68 in 1000 to 41 in 1000. However, the review also found an increased rate of perineal tearing in the induction group of 26 in 1000 compared to 7 in 1000 in the spontaneous labour group. The review also notes that “antenatal estimates of fetal weight are often inaccurate so many women may be worried unnecessarily, and many inductions may not be needed.” There are a number of risks associated with inducing labour (see this post and this post) that need to be considered against the potential risks of of birthing a big baby.
NICE guidelines and World Health Organisation guidelines both state that induction of labour should not be carried out simply because a baby is suspected of being big. Interestingly, Queensland Health induction guidelines do recommend induction if a baby is estimated to be big via ultrasound. Yet the Queensland Health gestational diabetes guidelines state that “estimation of fetal weight by clinical assessment or USS can have significant margins of error”.
Summary
The estimation of a baby’s size via ultrasound is inaccurate. The complications associated with big babies may reflect care provider fear and practice. International guidelines do not recommend induction for a suspected big baby. Women need to be given this information before agreeing to an ultrasound scan aimed at estimating the size of their baby. Once the ‘big baby’ label is applied it cannot be removed and may alter the birth experience and outcome.
As always, wonderful! Thank you !!!
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The hospital wanted me to have an extra scan to confirm my baby was “big” so they could induce me early. I declined, I ended up needing to be induced but I was nearly 42wks, had a 9lb baby (4080gms) small tear and minimal bleeding. My mum had had two cesars and smallish babies at term, but once my nana (on my dad’s side) told me she and my Aunty (her daughter) had “large” babies 3.9kgs and above I was relieved, I was never really stressed about it but she gave me a “good excuse” to argue my case. Everyone kept tell me I’d go early because I was “so huge” turns out I just grow healthy babies- he was 55cm long too
Love this article! My smaller baby was a hard birth, she was 3.005kg (6lb 10oz) and felt all elbows and knees 😉 labour was long and hard. My bigger baby was predicted (an emergency scan at 39+4) to be 4.819kg (10lb 10oz). I was terrified… the fear I went through that afternoon was immense. I went into labour that evening at around 11pm, she was born at 2am, labour was smooth and easy in comparison. She popped out a chubby 3.912kg (8lb 10oz) so a whole 2lbs heavier than her sister but also 2lb lighter than was predicted! I had no tearing, no pph and a really great recovery.
Thank you so much, brilliantly written!
Laura
it seems to me that the biggest problem is the estimation of baby’s weighy…. Why doing that if there is much imprecision and that brings fear?…. baby’s weight is one variable and in itself it says nothing about the dynamism of childbirth. We should always ask two questions: big for whom? compare to what?… childbirth is a complex phenomenon and should not be analyzed and managed with a mecanical perspective… Using only one variable in ordre to make decision can cause more harm than good… may be we need to use more wisdom in our practice?
Thank you for another informative and positive blog around women’s ability to birth the babies they grow!
I supported a woman to birth recently who had a beautiful 4.6kg baby girl. She was 42 weeks and 3 days gestational age. Mum had purposely avoided any US after 20 weeks as she wanted minimal intervention in her birth. She birthed like a goddess – strong, proud, fierce and beautiful. She did not experience a shoulder dystocia, probably due to the wonderful way she moved with her body and her baby in second stage to manage descent effectively and intuitively. She sustained no perineal damage (37cm head circumference) at all. I find it very disappointing every time I hear a woman has ‘a big baby on board’, my heart sinks in the knowledge that she has now been ‘tainted’ with the ‘big baby’ label. Given the poor accuracy of US to determine baby’s size we need to be extra careful to avoid labelling women and babies this way, and thereby giving women ‘the fear’ over their pending birth.
Hello Rachel,
Thank-you again for an informative article.
I wondered if there is research to quantify the statement you made in the opening: “…and each baby a woman has usually weighs more than the last.” How many subsequent babies are bigger and how much do they weigh?
Kind regards,
Rebecca
I am not aware of any specific research only looking at the size of babies. The findings are usually amongst other outcomes. And it is very individual… although usually within grams ie. not an entire kg
Here are some studies that support the statement that parity is associated with increased baby weight:
https://www.cambridge.org/core/journals/journal-of-developmental-origins-of-health-and-disease/article/maternal-predictors-of-neonatal-bone-size-and-geometry-the-southampton-womens-survey/AFE4865CDBF852AF913367DC4D6FDEAC
https://link.springer.com/article/10.1007/s11892-001-0013-y
https://www.sciencedirect.com/science/article/abs/pii/S0002937805000979
This was also my question. I suppose stats trump personal experience but I know others who share my experience of similar or decreasing weights of subsequent babies. Important to say to combat the care provider fear of ever increasing size!
My four eg: 8:13, 8:14, 8:12, 8:8
Individual women are not ‘stats’ and generalised stats often do not fit individual women or their experiences. Lots of women have smaller babies with subsequent pregnancies. I did myself 🙂
Just returning to this having forwarded the article to several women being invited to join n the UK’s Big Baby Trial; might you consider editing this blog in light of the above about subsequent babies getting bigger? I ask because it seems to perpetuate this belief often used to persuade women to accept induction/CB.
I have used the word ‘usually’ in the post.. and it is a true statement. I don’t think it helps to avoid the truth that for most women their next baby will be bigger than their previous one. This is important information for women to consider when making decisions. The issue remains that it is not the size of the baby that is the problem in most cases.
I just birthed my second big baby four weeks ago. My first daughter was born after 30 hours, in the hospital with epidural after my birthed stopped in the middle. She was 4,2kg, but no tearing on my side. My second daughter was born calmly in a midwife birthcenter, 10 hours labour, no tearing, relaxed baby, 4,3kg. For both pragnancies I choose the same midwife and the same OB Gyn, who are both very supportive in birthing a big baby normally. Birthing big babies without harm is obviously my natural super power.
As a doula who has served hundreds of clients, I have witnessed time and time again how “the perception of a baby’s size influences outcomes more than the actual size of the baby,” so thank you so much for drawing attention to this important issue.
I had two large baby births, both 6.5 – 7 hours of labour. Both were late for date. No shoulder dystocia, no GD. The first baby was 4.37 kgs, the second was 4.99kgs, minimal tearing. A previous birth was a still birth around 3.5kgs. I had two great midwives. The first was in an early hospital birth centre and the second was born at home.
This a wonderfully written post that is so needed on this topic! Thank you for putting the time into putting this together.
Hi! I’m 35 weeks pregnant and 5 days. Yesterday my doctor told me that I need an urgent pelvimetry as my baby seems to be a “big baby” actually they mentioned this since I was only 5 months pregnant. Yesterday he said that the atinares weight is 3.300 and his head is around 92mm.
Based on those numbers we thinks that maybe it will be easier to induce labor on week 37 and some days… to have a “vaginal birth” but I see impossible to guarantee that I will have one? Imagine inducing the labor and at the end they have to do an emergency c-section before
My complete term “40 weeks” I’m concern on what should I do, I’m also worry about the pelvimetry scan….
Pelvimetry is inaccurate and not evidence-based. I’m not sure what country you are in but it is rarely done. The pelvis opens during birth and can increase in size by around 30% (if in a position that allows movement ie. not reclining on a bed). Also note that 37 weeks is ‘early term’ and babies can have difficulty breastfeeding and long term health problems from early birth. Induction increases your chance of a c-section significantly. Can you find as supportive evidence-based care provider?
Currently being pushed into a frightening labour. I have large BMI and this will be my 3rd live child. No gd no other health issues. Was told at the beginning of my pregnancy that if I kept weight gain low I could be referred to mlu in the hospital. My 30 wk scan said she was 4lb my 34wk scan said 6lb. The consultant brought up the big baby study trial and said I was at risk of shoulder dystocia. That if I don’t have her by 38 wk app they will be discussing induction. I’ve been to the consultant midwife she recommends labour ward and was positive in discussion but printed out loads of leaflets that have scared the life out of me. I had two vaginal births had vontouse with first cos he pooped on way out so I don’t understand how this time around everything is so chaotic. This has helped me see that they are just trying to scare me and that it is seriously fucked up.
This is a fabulous post with great links to evidence. I recently had a CCE who had multiple growth and wellbeing scans during her pregnancy due to GDM, and was told at her 36 week scan that her baby was in the 99th centile. As a result of this (and the fact she was using Metformin and Insulin) she was booked for induction at 38+3… she did consent to induction but thankfully she laboured spontaneously on her own before this and had a beautiful, physiological birth with no complications. Her baby was not big at all… a healthy 3.6kg! No where near even the 90th centile…
This is a fabulous post with good links to evidence! I recently had a CCE that was offered multiple growth and wellbeing scans throughout her pregnancy as she had GDM (treated with Metformin and Insulin). At the 36 week scan they told her that her baby was measuring in the 99th centile. For this reason, and the fact she had GDM, they booked her for induction at 38+3. She did ‘consent’ to this, but thankfully laboured spontaneously before her induction date and had a beautiful birth with no complications. Her baby was not big at all… a very healthy 3.6kg. However, during the labour, she needed so much reassurance because all she could think was “what if this baby is too big for me to push out?”. The midwife TL of the birth suite this day also demanded that I shouldn’t be the accoucheur for the birth because “this baby is going to be big”. It was really disappointing for me that she was filled with so much fear and that this midwife really fed into that.
On another note – I often see women on placement being induced for “GDM Metformin and/or Insulin”, however I cannot find any evidence to support this, even Queensland Health guidelines do not recommend this (however no link to evidence). Have you come across anything in your teaching/research?
Yes – practice is often culture-based rather than evidence-based. There is no evidence for induction if BGLs are normal.
Really interesting article. My baby boy weighed 4.06kg at birth and had a head circumference of 37.5cm (98th centile). Unfortunately I suffered lots of complications: I had a prolonged induction under syntocin after a pre-labour rupture of membranes at 41 weeks (induction was required due to infections risks they said… and tbf I already had thrush at the time and didn’t want to risk it. I stayed at home over 24hrs to see if labour would start spontaneously but it didn’t so I gave in and had an induction). At that point I wasn’t worried about the size of my baby. Fast forward to the pushing phase after 14hrs of syntocin and about 8 hours of epidural I couldn’t push my son out. I tried for 2 hours to no avail. I was exhausted after a first all nighter and this was going to be my second night without sleep. I ended up requiring forceps, episiotomy and losing 2.7 litres of blood. I was told the haemorrhage came from both the episiotomy incision and multiple internal tears, due to the size of the baby’s head and prolonged use of syntocin potentially affecting the tissues. 18 months on, I still have incontinence (leaking urine when I try to jump or run) despite months of physiotherapy (disrupted by the pandemic of course). And this is affecting me deeply, putting me of trying for a second baby. I keep wondering what went wrong, what I might have done differently and sometimes wonder if the size of my baby was a big factor in my poor outcomes and recovery. I’ve had a debrief with the hospital and the only thing they agreed we could have done differently was to shorten the pushing phase. I’m not sharing this to scare people off but just wondering if size can matter in some cases? Unless this is all down to the induction? Any reflections that would help people in my case know what to do with my next pregnancy/birth?
Unfortunately, your experience is common regardless of the size of the baby 🙁
Thank you Rachel! I have a CCE woman wanting a VBAC, but now that the spectre of ‘big baby’ has been raised the obstetrician will not let her go past 40 weeks. She is booked in for a c-section if she hasn’t managed to labour naturally before then. He reckons her cervix is not favourable for an induction…
You’re right, women should be warned of the risks before they have an ultrasound for suspected ‘big baby’. Unfortunately I was unable to be at a couple of important appointments 🙁 . Not sure if I would have dared to question the ultrasound referral even if I had been there….
He can’t legally do anything without her consent. So it is not up to him to ‘leg her’ do anything with her body.
Hi, Dr. Rachel!
Thank you for such an interesting article. I really like how you pointed out that just because one thing is associated with another does not mean it’s the cause. Correlation does not always mean causation. I enjoyed the example of ice cream sales and shark attacks in your other article.
Thank you Dr Rachel.
I was induced at 38 weeks with my first child as he was measuring ‘big’, expected to be over 10lbs. It was a long & traumatic experience and he was born a healthy 8lbs. I still feel annoyed that this inaccuracy led me to have an intervention and increased stress during my pregnancy & birth. With my second pregnancy I was again told my baby was measuring big during 2 ultrasounds, and when I recounted my first experience I felt the ultrasound technicians became annoyed with me and my midwife discounted my concerns about the inaccuracies. Thank you for writing this article and validating for me that there is research to show my negative experience wasn’t necessary. It is healing for me to know this!