Estimated fetal weight: Why Ultrasound Numbers Have a Margin of Error

Estimated fetal weight is a calculation made from ultrasound measurements. It is often shortened to EFW on a scan report. The number helps your team assess size and growth, but it is not an exact weight or a guaranteed prediction of the weight at birth.

Where the estimate comes from

Measurements such as head circumference, abdominal circumference and thigh bone length are used in a formula. The result is compared with a chart for the pregnancy stage. A centile describes the baby’s estimated size relative to the reference group; it is not a percentage score for health.

Ask which chart is being used and whether the discussion concerns the weight estimate, one particular measurement or a change over time. The growth scan guide explains the other observations that can accompany the calculation.

Understand the margin of error

Ultrasound weight estimates have meaningful uncertainty. Some clinical guidance describes a margin of about 15%, and some hospital information describes a wider range. Accuracy varies, so do not treat one percentage as a guaranteed boundary for an individual baby.

For illustration, 15% of a 3 kg estimate is 450 g. That shows why an apparently precise number can still involve a substantial difference. It does not establish your baby’s true weight, and further growth between the scan and birth also changes the comparison.

Ask the clinician how uncertainty affects the decision being discussed. A question such as “Would the recommendation change if the true weight were somewhat lower or higher?” is often more useful than recalculating the figure repeatedly at home.

Look at the wider picture

Size and growth are related but different. Clinicians may consider the pattern across scans, abdominal measurements, fluid and blood flow findings, alongside your health and pregnancy history. Some small babies are healthy; others need closer assessment because they are not reaching their growth potential.

Our guide to a baby measuring small explains that distinction. A single centile should not be used on its own to decide that everything is well or that a complication is certain.

Keep decisions specific

If a weight estimate leads to a birth discussion, ask what options are being offered, which risks apply to you and what further information could help. Bring the actual report to the conversation rather than relying on a remembered number.

Keep the date of each scan beside its result. Ask your team to explain changes rather than trying to compare numbers taken at different pregnancy stages as though they were measurements made on the same day.