Fetal Growth Calculator
Enter ultrasound biometry to estimate fetal weight (Hadlock formula) and see how it compares to gestational-age norms.
Educational estimate only. This is not a diagnosis. Ultrasound-based weight estimates carry a margin of error of roughly ±15%; always rely on your provider’s report and clinical judgment.
Compare to a previous scan (optional)
Enter HC, AC, FL and gestational age to see your estimate.
How fetal growth is estimated from ultrasound
Quick answer: Sonographers estimate fetal weight from head circumference (HC), abdominal circumference (AC) and femur length (FL) using the Hadlock formula, then compare that estimate to a reference chart for the same gestational age. A result below the 10th percentile is called small for gestational age (SGA); above the 90th percentile is large for gestational age (LGA); in between is appropriate for gestational age (AGA).
The Hadlock formula, published in 1985 and still the most widely used method worldwide, combines biometric measurements in a regression equation to produce an estimated fetal weight (EFW) in grams. Adding a fourth measurement, biparietal diameter (BPD), slightly improves accuracy over the three-parameter version. No formula is exact — most published estimates fall within about 15% of actual birth weight, and accuracy tends to drop for very small or very large babies.
A single EFW is a snapshot, not a trend. Two estimates a few weeks apart that show a fetus tracking steadily along its own growth curve are generally more reassuring than one low or high number in isolation. A percentile that drops sharply between scans, even while still technically in a “normal” range, is one of the patterns providers watch for most closely.
Fetal weight by gestational age (10th–90th percentile)
| Week | 10th %ile | 50th %ile (approx.) | 90th %ile |
|---|---|---|---|
| 20 | 289 g | 332 g | 381 g |
| 22 | 417 g | 479 g | 551 g |
| 24 | 583 g | 670 g | 771 g |
| 26 | 791 g | 911 g | 1050 g |
| 28 | 1045 g | 1206 g | 1391 g |
| 30 | 1343 g | 1554 g | 1799 g |
| 32 | 1686 g | 1959 g | 2276 g |
| 34 | 2058 g | 2404 g | 2809 g |
| 36 | 2432 g | 2861 g | 3368 g |
| 38 | 2777 g | 3299 g | 3918 g |
| 40 | 3080 g | 3702 g | 4450 g |
Reference values derived from the NICHD Fetal Growth Studies (Buck Louis et al., 2015). This calculator uses the same dataset, interpolated week by week, from 20 to 40 weeks.
Frequently asked questions
What is the Hadlock formula?
It’s a regression equation that estimates fetal weight from ultrasound measurements of the head, abdomen and femur (and optionally the biparietal diameter). Published in 1985, it remains the most widely used method for estimating fetal weight.
How accurate is ultrasound estimated fetal weight?
Most estimates fall within about 15% of actual birth weight, but accuracy decreases at the extremes — very small or very large babies are harder to estimate precisely, and results can shift with fetal position or measurement technique.
What does SGA mean?
Small for gestational age (SGA) means the estimated weight is below the 10th percentile for that gestational age. Many SGA babies are healthy and simply constitutionally small; some have true growth restriction and need closer monitoring.
What does LGA mean?
Large for gestational age (LGA) means the estimated weight is at or above the 90th percentile. This can be constitutional or linked to factors like gestational diabetes, and may affect delivery planning.
Why might my baby’s percentile change between scans?
Some fluctuation is normal because each estimate carries measurement error. A consistent drift across two or more scans is more meaningful than any single reading, which is why providers look at growth trends rather than one number.
Which growth reference does this calculator use?
It uses the Hadlock formula for weight estimation and percentile ranges derived from the NICHD Fetal Growth Studies. Other valid references exist, such as INTERGROWTH-21st, and may give slightly different percentiles.
Can an EFW percentile alone diagnose growth restriction?
No. A low percentile prompts further evaluation — such as Doppler studies and amniotic fluid assessment — rather than a diagnosis on its own. Many small-for-gestational-age babies turn out to be healthy.
Does this tool replace my doctor’s ultrasound report?
No. This is an educational estimate for general understanding. Only your provider’s full ultrasound report, growth history and clinical assessment can properly evaluate fetal growth.