Being told your baby is measuring bigger or smaller than expected can be worrying, but in most cases it's not a cause for alarm. Here's what these measurements really mean.
Babies Come in Different Sizes
Just like children and adults, babies grow at their own individual rates. A measurement slightly above or below the average is usually just natural variation — most "big" or "small" babies are perfectly healthy.
Sonographers estimate size by measuring the head, abdomen, and thigh bone, then combining these into an estimated weight and a percentile. A percentile simply shows how your baby compares to others at the same stage.
Common Reasons for Measuring "Big"
- Natural size — some babies are simply larger, often running in families
- Dates being slightly off — if the pregnancy is further along than thought
- Gestational diabetes — which can increase growth, and is one reason it's screened for
Common Reasons for Measuring "Small"
- Natural size — smaller parents often have smaller babies
- Dates being slightly off — if the pregnancy is earlier than thought
- The baby's position making measurement harder
- Less commonly, issues with the placenta or blood flow, which is why follow-up matters
Why One Measurement Isn't the Whole Story
A single scan is a snapshot. What matters more is the trend over time — whether the baby is growing steadily along its own curve. This is why a "big" or "small" result often leads to a repeat growth scan a few weeks later, rather than immediate concern.
What Your Care Team Will Do
Depending on the finding, they may:
- Arrange a follow-up growth scan
- Check blood flow with a Doppler study
- Screen for gestational diabetes
- Simply reassure you if everything else looks normal
Try Not to Worry
Estimated fetal weight from ultrasound has a margin of error of around 10–15%, so it's an estimate, not an exact figure. Your care team interprets it alongside everything else they know about your pregnancy.
This article is for general information only and is not a substitute for professional medical advice.
Ready to Book Your Ultrasound Scan?
Mascot Healthcare Clinic, Akoka (Yaba), Lagos offers ultrasound scans by appointment only, Monday–Saturday, 9am–5pm. Book online and choose a time that works for you.