Fetal Weight Estimator

How scans estimate weight

Ultrasound cannot weigh a baby. What it does is measure several body dimensions and put them into a regression formula derived from babies whose measurements and birth weights were both recorded.

The measurements typically used are:

• BPD — biparietal diameter, across the head
• HC — head circumference
• AC — abdominal circumference
• FL — femur length

The most widely used formula is Hadlock, which combines these into an estimated fetal weight. Abdominal circumference carries the most weight in the calculation, since it reflects soft tissue and liver size, which track nutritional status.

How accurate the estimate is

This is the part worth knowing before a growth scan. Estimated fetal weight typically carries a margin of ±10 to 15%.

Practically, an estimate of 7 lb means the baby is probably somewhere between 6 lb and 8 lb. At the upper end of pregnancy, that is a difference of well over a pound — enough to change how a number feels without changing anything real.

Accuracy is worse at the extremes: estimates tend to be less reliable for very large and very small babies, and for pregnancies with low amniotic fluid, a high maternal BMI, or a baby in an awkward position.

This matters because scan estimates are sometimes used to guide decisions about induction or delivery method. A number carrying a pound of uncertainty is one input among several, not a measurement.

Average weights by week

Rough averages for a single baby, though individual variation is enormous:

• 20 weeks: around 300 g (11 oz)
• 24 weeks: around 600 g (1 lb 5 oz)
• 28 weeks: around 1,000 g (2 lb 3 oz)
• 32 weeks: around 1,700 g (3 lb 12 oz)
• 36 weeks: around 2,600 g (5 lb 12 oz)
• 40 weeks: around 3,400 g (7 lb 8 oz)

Weight gain accelerates sharply in the third trimester — babies typically gain around half of their birth weight in the final ten weeks, laying down fat that supports temperature regulation after birth.

Percentiles and what they trigger

Rather than the raw number, clinicians look at where a baby sits relative to gestational age.

Below the 10th percentile is described as small for gestational age. This may simply mean a constitutionally small baby — small parents often have small babies — or it may indicate growth restriction, which needs monitoring. Serial scans distinguishing between the two are more informative than any single measurement.

Above the 90th percentile is large for gestational age. This is associated with maternal diabetes, including gestational diabetes, and can influence delivery planning.

In both cases the trend across scans matters more than any one estimate — a baby tracking steadily along the 8th percentile is usually reassuring, while one falling from the 50th to the 10th prompts closer attention.

A note on how this lands

Being told a baby is measuring large or small is stressful, and the stress is often out of proportion to what the number can actually tell you. Given a ±15% margin, many babies flagged as large or small at scan are born entirely within the normal range.

If you have had a growth scan and are worried about the result, ask your midwife or obstetrician what it means for your care specifically, and whether a repeat scan is planned. A single estimate rarely determines anything on its own, and the people looking after you can see the whole picture — your history, your measurements, the amniotic fluid, and the blood flow — that a weight figure cannot capture.

Common Questions