Been Told your Baby is Measuring Big: Why Size Predictions in Pregnancy Often Get It Wrong.
- Joy
- Sep 17
- 6 min read
Updated: Sep 30
If you’re pregnant in Edinburgh, West Lothian, Falkirk or the wider Forth Valley, chances are you’ve already heard the phrase: “Your baby is measuring big.”
It sounds harmless, but for many parents it’s the start of what I call the curse of the big baby. A casual comment can spark anxiety, chip away at body confidence, and lead families down a road of interventions such as induction, caesarean, increased monitoring ... only for the baby to arrive at a perfectly average weight.
So let’s unpick what’s really going on when we talk about “big babies”.
What’s considered a “big baby”?
In the UK, a baby is usually considered large for gestational age if they weigh more than 4kg (8lb 13oz) at birth. Some studies use 4.5kg (9lb 15oz) as the cut-off. Remember: big doesn’t automatically mean unhealthy - many large babies are born perfectly safely.
Are big babies actually that unusual?
In well-nourished countries like the UK, around 1 in 10 babies are born weighing 4kg (8lb 13oz) or more. Genetics play a role too - bigger babies often run in families, and each baby a woman has tends to weigh a little more than the last.
A baby born at 42 weeks will almost always be bigger than the same baby born at 40 weeks - that’s simply how growth works. The important point? A “big” baby isn’t necessarily a problem.
When big babies may be a concern
There are situations where a larger baby may bring more risk. For example, babies who grow bigger because of uncontrolled gestational diabetes can carry weight differently - often with broader shoulders and chest - which may increase the risk of shoulder dystocia (when the baby’s shoulders get stuck during birth). But, here’s where it gets tricky: research often mixes together babies born to mothers with diabetes and those without, which makes the risks look higher than they are for the average pregnancy.
How accurate are ultrasound weight estimates in pregnancy?
Here’s the heart of it: ultrasound scans and bump measurements are not an exact science.
Tape measure (symphysis-fundal height): affected by your body shape, fluid levels, and how the baby is lying.
Ultrasound estimates: often out by 10–15%. That’s the difference between being told your baby might weigh 9lb… and them being born at 7lb 10oz.
One large study found that half of all babies estimated to weigh over 4kg by ultrasound were not actually that big at birth. Another found that a third of women told their baby was “too big” gave birth to babies under 8lb.
Put bluntly: the only way to know a baby’s size for certain is to weigh them after they’re born.
The impact on birth choices
The “big baby” label often changes the course of care. Parents may suddenly find themselves offered:
Induction of labour - sometimes weeks before their due date
Elective caesarean - out of fear that labour will be “too difficult”
Restrictions on birth choices - such as not being “allowed” a water birth or midwife-led setting
“Women suspected of carrying a big baby are three times more likely to be induced or have a caesarean - even if their baby is average size.”【Little et al., 2012】
“Four times more complications occur in women who were told their baby was big compared to those who birthed a large baby unexpectedly.”【Sadeh-Mestechkin et al., 2008】
Research shows that suspected big babies are more likely to lead to interventions, not necessarily because of actual size, but because of care provider concern. In fact, women labelled with a big baby are three times more likely to be induced or have a caesarean, and four times more likely to experience complications linked to those interventions.
The result? Parents making decisions out of fear, not informed choice.
The effect on mums’ mental state
This isn’t just about medical decisions - it’s about confidence and emotional wellbeing too.
Being told your baby is “too big” can:
Make you doubt your body’s ability to give birth
Heighten fear of labour and birth
Increase feelings of failure or blame if things don’t go “to plan”
Lead to anxiety about food, weight gain, or body image during pregnancy
I’ve supported parents across West Lothian and Falkirk who’ve described feeling scared, ashamed, or pressured when the big baby label was applied. And for some, that worry lingered into the postnatal period, affecting bonding and confidence as a new parent.
Your mental wellbeing matters just as much as your physical health. A label should never undermine that.
What about shoulder dystocia?
Yes, larger babies are associated with a higher chance of shoulder dystocia, but association isn’t the same as cause. Most shoulder dystocias happen with average-sized babies. And interventions like early induction don’t reliably prevent it - what matters more is skilled, supportive care in the moment.
Big baby myths that need busting
Let’s get rid of a few unhelpful myths:
Myth: You can’t birth a big baby vaginally. Fact: Many people safely do, every single day.
Myth: A big baby means you will definitely tear. Fact: Tears can happen with babies of all sizes. Good birth support makes the biggest difference.
Myth: A caesarean is always safer if your baby is big. Fact: Every option carries risks and benefits - decisions should be based on your individual circumstances, not assumptions.
Questions to ask if you’re told your baby is “measuring big”
Here are some calm but powerful questions you might ask your midwife or doctor:
What’s the margin of error with this measurement?
How often do suspected “big babies” here actually turn out to be over 4kg / 8lb13oz?
What are the specific risks of waiting for labour to start naturally versus induction?
How do risks of intervention compare with risks of doing nothing?
Can I still choose where and how I give birth?
Can I have time to consider my options before deciding?
These questions put the power back in your hands and help you make choices that feel right for you.
How birth partners can help
If you’re supporting someone through pregnancy, your role is vital. Partners can:
Help ask the tough questions at appointments
Be the calm voice when fear-based language crops up
Remind mums of their strength and options
Back their choices, whatever they are
Empowered partners mean empowered families.
FAQ: Big Baby Pregnancy in Scotland
Can a big baby be delivered naturally? Yes. Many large babies are born vaginally with no complications. Support and position changes during labour can make a huge difference.
How accurate is an ultrasound at predicting baby size? Not very. Estimates are often out by up to 15%. Half of babies predicted to be “big” are average size at birth.
Do I need a caesarean if my baby is large? Not automatically. Decisions should be based on your whole situation, not just an ultrasound estimate.
Should I be induced if my baby is measuring big? Induction has pros and cons. It may lower certain risks, but increases others. Ask your care provider about the evidence and your options.
A local perspective
Here in Linlithgow, West Lothian, I hear the same story often: parents left anxious because they’ve been told their baby is “too big.” That’s why in my antenatal courses in Linlithgow, designed for families across Edinburgh, the Lothians and Forth Valley, we spend time unpacking these scenarios, looking at the evidence, and practising ways to advocate for yourself in appointments.
When parents leave with confidence, they’re less likely to feel pressured into something that doesn’t sit right with them.
Final thought
The curse of the big baby isn’t about babies at all. It’s about how information is shared, and how it makes parents feel.
You deserve more than scare tactics. You deserve clear information, balanced evidence, and the confidence to say: “I’ll decide what’s right for me and my baby.”
If you’re pregnant in Edinburgh, West Lothian, Falkirk or the Forth Valley, and want calm, evidence-based antenatal education (without the scare stories), my in-person antenatal courses in Linlithgow could be just what you need.
References
NHS (2022). Your pregnancy and baby guide. National Health Service UK. https://www.nhs.uk/pregnancy/
Royal College of Obstetricians & Gynaecologists (2019). Large-for-Gestational-Age Baby (Green-top Guideline No. 42). RCOG. https://www.rcog.org.uk/guidance/browse-all-guidance/green-top-guidelines/large-for-gestational-age-green-top-guideline-no-42/
Boulvain, M., Senat, M., Perrotin, F., et al. (2015; updated 2021). Induction of labour versus expectant management for large-for-date fetuses: the Big Baby Trial. The Lancet, 385(9987), 2600–2605.
Henrichs, J., Verfaille, V., Jellema, P., et al. (2019). Accuracy of estimated fetal weight in predicting macrosomia: a systematic review and meta-analysis. Ultrasound in Obstetrics & Gynecology, 54(6), 722–731.
Little, S. E., Edlow, A. G., Thomas, A. M., & Smith, N. A. (2012). Estimated fetal weight by ultrasound: how accurate is it? And what are the clinical consequences? American Journal of Perinatology, 29(9), 705–712.
Sadeh-Mestechkin, D., Yogev, Y., Ben-Haroush, A., et al. (2008). Suspected macrosomia? Better not tell. Archives of Gynecology and Obstetrics, 278(3), 225–230.
Cochrane Pregnancy and Childbirth Group (2016). Induction of labour for suspected fetal macrosomia. Cochrane Database of Systematic Reviews.
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