Fetal Biometry: Complete Guide to Pregnancy Measurements
During pregnancy ultrasounds, the sonographer takes measurements of your baby called fetal biometry. These numbers, plotted against standard growth charts, tell your healthcare team whether your baby is growing as expected for their gestational age. This guide walks through what each measurement means and how to read your scan report.
The Main Measurements Explained
Every biometry scan focuses on a small set of standard measurements, each giving a slightly different picture of growth.
- Crown-rump length (CRL) — measures your baby from head to bottom in the first trimester (roughly 7 to 14 weeks). It's the single most accurate way to date a pregnancy, more reliable than counting from your last period. After around 14 weeks the baby curls up too much for CRL to stay accurate, so other measurements take over.
- Biparietal diameter (BPD) — the width of the head from side to side, used from the second trimester onwards to estimate gestational age and track head growth.
- Head circumference (HC) — the full circumference of the head, used alongside BPD, particularly useful if the head shape is slightly unusual (which can throw off BPD alone).
- Abdominal circumference (AC) — measured around the tummy at the level of the stomach and liver. AC is especially sensitive to nutrition, so it's often the first measurement to show a problem if growth is slowing.
- Femur length (FL) — the length of the thigh bone, used to check limb growth and add another data point for dating and growth assessment.
Estimated Fetal Weight and Growth Centiles
Sonographers combine BPD, HC, AC and FL using a standard formula to calculate an estimated fetal weight (EFW), plotted on a centile chart alongside any earlier scans.
- A centile shows where your baby sits compared to other babies at the same gestational age — the 50th centile is exactly average.
- Measurements below the 10th centile may suggest your baby is small for gestational age, which can sometimes (not always) indicate reduced growth.
- Measurements above the 90th centile may suggest a larger baby, which your team will factor into discussions about labour and birth.
- A single measurement matters less than the trend across several scans — steady growth along a centile line is usually more reassuring than one number in isolation.
How Results Are Used
If any measurement falls outside the expected range, this doesn't automatically mean something is wrong. Dating can be slightly off, babies grow at individual paces, and measurement itself has a small margin of error built in.
- Your sonographer or midwife will explain what the numbers mean for your specific scan.
- A follow-up growth scan two to three weeks later is the usual next step if there's uncertainty, so your team can see the trend rather than a single snapshot.
- Additional checks, such as Doppler blood flow studies of the umbilical cord, may be offered if growth restriction is suspected.
- Most babies with a borderline measurement at one scan go on to have entirely normal growth on the next.
Frequently Asked Questions
What is fetal biometry?
Fetal biometry is the set of standard measurements taken during an ultrasound — including CRL, BPD, HC, AC and FL — used to estimate gestational age and monitor your baby's growth throughout pregnancy.
Which measurement is most accurate for dating my pregnancy?
The crown-rump length (CRL), taken between roughly 7 and 14 weeks, is considered the most accurate single measurement for confirming your due date.
What happens if my baby measures small on a scan?
Your healthcare team will usually book a follow-up growth scan a few weeks later to check the trend, and may add extra checks like Doppler blood flow studies if there's ongoing concern. A single small measurement is common and often resolves on the next scan.
This article is for general information only and does not replace advice from your midwife, doctor, or sonographer.
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