Fetal Weight Estimator
Look up the average fetal weight for any week from 10 to 40 using the Hadlock 1991 ultrasound weight standard, with the typical 10th to 90th percentile range.
Last updated: October 2026
Average fetal weight for this week (50th percentile)
1,953 g
Formula below · 3 sources (pubmed.ncbi.nlm.nih.gov, obgyn.utoronto.ca, opqic.org) · Updated Oct 2026
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About this calculator
This calculator gives the average (50th percentile) weight of a fetus at a given week, from the Hadlock 1991 sonographic weight standard:
ln(weight in g) = 0.578 + 0.332 × week − 0.00354 × week²
Hadlock built the curve from ultrasound weight estimates in 392 pregnancies and checked it against birth weights. It runs from about 35 g at 10 weeks to about 3,620 g at 40 weeks, and the spread around the average is about 12.7% either way (one standard deviation). Using that spread, the 10th to 90th percentile range is roughly the average ± 16%; the table below lists it for several weeks.
This is a population average for the week, not an estimate of your own baby's weight. For that, a clinician uses ultrasound measurements of the head, abdomen and thigh bone; the Fetal Growth Calculator does that from BPD, AC and FL. Fetal weight estimates and percentiles are interpreted by your care team. This page is for information, not medical advice.
| Week | Average (g) | Average (lb, oz) | About 10th–90th percentile (g) |
|---|---|---|---|
| 12 | 58 | 0 lb 2 oz | 48–67 |
| 16 | 146 | 0 lb 5 oz | 122–170 |
| 20 | 331 | 0 lb 12 oz | 277–385 |
| 24 | 670 | 1 lb 8 oz | 561–779 |
| 28 | 1,210 | 2 lb 11 oz | 1,013–1,408 |
| 32 | 1,953 | 4 lb 5 oz | 1,635–2,271 |
| 36 | 2,813 | 6 lb 3 oz | 2,355–3,272 |
| 40 | 3,619 | 8 lb 0 oz | 3,029–4,209 |
How to use
Enter the week of pregnancy from 10 to 40. Decimals work: 32.5 is 32 weeks and 3 to 4 days. Example (the default): at 32 weeks, ln(weight) = 0.578 + 0.332 × 32 − 0.00354 × 32² = 7.577, so the average weight is e^7.577 ≈ 1,953 g (4 lb 5 oz). The approximate 10th to 90th percentile range is 1,953 × 0.84 to 1,953 × 1.16, about 1,635 to 2,271 g. Other weeks: 20 weeks gives 331 g, 25 weeks 785 g, 36 weeks 2,813 g and 40 weeks 3,619 g (8 lb 0 oz). To convert grams to pounds, divide by 453.6.
Frequently asked questions
How much does a baby weigh at each stage of pregnancy?
On the Hadlock standard the average is about 35 g at 10 weeks, 331 g at 20 weeks, 1,210 g at 28 weeks, 2,813 g at 36 weeks and 3,619 g at 40 weeks. The table below lists more weeks.
Is this my baby's estimated weight?
No. It is the average for the week across many pregnancies. Your baby's estimated weight comes from ultrasound measurements, and healthy babies range well above and below the average.
What do the 10th and 90th percentiles mean?
About 10% of fetuses weigh less than the 10th percentile for their week and 10% weigh more than the 90th. ACOG uses an estimated weight below the 10th percentile as the definition of fetal growth restriction, which leads to closer monitoring; above the 90th percentile is called large for gestational age.
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