Growth scan
Measures the baby's head, abdomen and thigh bone to estimate weight and check that growth is following its own curve over time.
When: Usually from 28 weeks, repeated as advised
Examination — looks at how things are
Also called: Fetal growth scan · Third-trimester scan · Biometry scan
What it is
A growth scan measures a few specific parts of the baby — the head, the circumference of the abdomen, and the length of the thigh bone — and combines them into an estimated weight.1
Growth is a trend, not a single number. One measurement tells you the baby's size today; it takes two or more, spaced apart, to show whether the baby is growing steadily. This is why growth scans are repeated rather than done once, and why a single reading below or above average is rarely conclusive on its own.1
The scan also checks the fluid around the baby and, where the doctor asks for it, blood flow in the cord and placenta.
Estimated weight is an estimate
Ultrasound weight estimates carry a genuine margin of error, and that margin is wider the bigger the baby is. It is used to spot babies growing much less or much more than expected — it is not a prediction of birth weight, and it should not be read as one.1
Why a small baby is worth finding
A baby growing more slowly than expected may simply be a small baby, or may be one the placenta is not supporting well. Telling those apart changes how closely the pregnancy is monitored and sometimes when delivery is planned — which is the whole reason growth is tracked in the third trimester.
Why a large baby is worth finding
A baby measuring large raises questions about blood sugar in pregnancy and about the plan for delivery. Knowing in advance means it can be discussed rather than met unprepared on the day.
No scan or screening test can rule out every condition. What each one looks at, what it found, and what it means for you is a conversation with the doctor who performed it — this page explains the tests, it does not interpret your results.
References
- Salomon LJ, Alfirevic Z, Da Silva Costa F, Deter RL, et al.. ISUOG Practice Guidelines: ultrasound assessment of fetal biometry and growth. Ultrasound in Obstetrics & Gynecology. 2019;53(6):715-723. PMID 31169958. doi:10.1002/uog.20272The international guideline on growth scans — how fetal size is measured and why growth is judged over time rather than from a single scan.
Sources are listed for the specific statements they support. They are published research and guidance, not a substitute for a consultation — what applies to you is decided by a doctor who has assessed you.