From Measurements to Weight
Ultrasound biometry measures the biparietal diameter, head circumference, abdominal circumference, and femur length. The Hadlock equation, validated on thousands of pregnancies, combines them into an estimated fetal weight.
The estimate is honest about its limits: ultrasound weight is typically within ±10–15% of birth weight, enough to guide care but not to promise a specific birth size.
What Percentiles Mean
The percentile ranks the estimate against the distribution for the same gestational week. Between the 10th and 90th is typical; below the 10th raises the possibility of growth restriction; above the 90th suggests a large-for-gestational-age baby.
A single low percentile isn't a diagnosis, it's an invitation to look closer. Serial scans showing a falling trajectory, plus Doppler studies of placental blood flow, are what actually establish growth restriction.
Questions Worth Asking
At each scan, ask: what percentile is the overall estimate, has the trajectory changed, and are the amniotic fluid and placenta normal? Those answers matter far more than any single measurement.
And remember the reference population matters, growth charts differ slightly by country and ethnicity, which is why your unit's chart may not match an app's.
Sources
FAQ
What percentile is normal for fetal weight?
10th–90th is typical. Below the 10th suggests possible growth restriction; above the 90th suggests a large baby.
How accurate is estimated fetal weight?
Within about ±10–15% of birth weight, accurate enough for care decisions, imprecise enough that single scans shouldn't be over-read.
What if my baby measures small?
Providers repeat scans to check the growth trend and assess placental blood flow before diagnosing growth restriction.
A note before you begin: this tool offers educational estimates only. It cannot replace advice from your doctor or another qualified professional.