Why do some people have big babies?
First, let’s clear something up:
👉 A ‘big baby’ is not automatically a problem.
Many larger babies are born beautifully and without complication. Baby size is just one part of a much bigger picture.
What do we mean by a “Big Baby”?
Medically, a baby might be described as “large for gestational age” or “macrosomic”, which usually means weighing around 4kg (8lb 13oz) or more at birth. But in everyday life, “big” is very subjective — one person’s big baby is another’s perfectly average one!
And importantly, scans can only ever estimate size — they are not crystal balls.
So… What Actually Makes Some Babies Bigger?
There isn’t one single reason. Baby size is influenced by a lovely mix of biology, genetics and how pregnancy unfolds.
Here are the most common factors:
1. Blood sugar and gestational diabetes
If a parent has gestational diabetes or higher blood sugar levels, more sugar crosses the placenta to the baby. Babies respond by producing more insulin, which encourages growth — a bit like a fertiliser effect.
This is one of the strongest links we see in research, but it’s also something that can usually be well managed with the right care and support.
And importantly — having gestational diabetes does not mean something has gone wrong. It’s very common and very treatable.
2. Body weight and weight gain in pregnancy
People who start pregnancy with a higher BMI or who gain more weight than average during pregnancy are statistically more likely to have larger babies.
But this is not about blame or “doing pregnancy wrong”. Bodies are all different, and weight is influenced by hormones, metabolism, genetics and life circumstances — not just willpower or food choices.
3. Genetics: family traits matter
Sometimes babies are big simply because… that’s what your family does!
If you, your partner or close relatives were larger at birth, or are generally tall or broad-built, it makes sense that your baby might follow that pattern. Our bodies pass down more than just eye colour!
4. How long baby stays on the inside
Babies who arrive after their due date often weigh a bit more — simply because they’ve had longer to grow.
Also, second and subsequent babies are often a little bigger than first babies. Your body is already familiar with the process.
5. A few other influences
Some other things linked to bigger babies include:
- Having a boy (they’re slightly more likely to be bigger)
- Being over 35
- Having previously had a large baby
And sometimes… there is no clear reason at all. Some babies are just built bigger — and that’s perfectly normal.
What doesn’t really cause big babies
Let’s bust a common myth:
🍰 Eating the odd slice of cake does not create a giant baby.
There is no evidence that specific foods directly make babies big — unless they affect overall blood sugar or weight gain patterns. So please don’t feel guilty about enjoying your food in pregnancy.
Why does any of this matter for birth?
Knowing a baby might be larger can help your care team and your birth preparation focus on:
- optimal positioning
- calm, supported birthing phase
- and being aware of how to support baby’s shoulders at birth
But it does not mean you can’t have a calm, positive, vaginal birth — many people do.
In hypnobirthing, knowledge is power: not to create fear, but to build confidence and choice.
A final thought
Baby size is influenced by many things — and most of them are not fully within your control. And that’s okay. Predicting your baby’s size before they are born can be inaccurate (up to 10%) so don’t let growth scans freak you out.
What matters most is:
✨ feeling informed
✨ feeling supported
✨ and trusting your body while staying open to guidance
Big babies can be born beautifully. Small babies can need support. There is no “perfect” size — only the baby that is right for you.