15/02/2026
Did you know that ultrasound scans in late pregnancy can be wrong by up to 10–15% either way when estimating baby’s weight?
That means if your scan says your baby is 9lbs…
They could actually be 7lb 10oz.
Or over 10lbs.
In fact, research shows that estimated fetal weight in the third trimester has a significant margin of error, especially at the higher and lower ends.
And here’s something important:
👉 Ultrasound doesn’t measure your baby’s weight.
It measures parts of your baby — head, abdomen, femur — and then applies a formula.
That formula was developed decades ago…
Based on population averages.
Not your body. Not your baby.
Studies have shown that suspected “big babies” are frequently overestimated, and many babies predicted to be large are born within average weight ranges.
And once a label like “big baby” is used…
It can influence decisions around induction and caesarean birth.
✨ Remember:
• Scans are a useful tool
• But they are estimates — not guarantees
• Your body has grown this baby beautifully
Ask:
“How accurate is this estimate?”
“What’s the margin of error?”
“How would care change if we didn’t know this number?”
Because information should empower you — not pressure you.
