Fetal Weight Percentile Calculator
Use this fetal weight percentile calculator to see where your baby's ultrasound weight estimate falls compared with other babies at the same week.
Updated October 21, 2026
Medical Disclaimer: This calculator provides a general estimate based on established research and is for educational purposes only, not a substitute for medical advice. Individual results can vary based on health conditions, so consult a qualified healthcare professional or registered dietitian before making significant changes to your diet, exercise, or health routine. Read our full Medical Disclaimer.
How Fetal Weight Percentile Is Calculated
A fetal weight percentile compares your baby's estimated fetal weight (EFW) from an ultrasound scan with other babies at the same week of pregnancy. A result at the 30th percentile means about 30 in 100 babies at that week are estimated to weigh less, and about 70 weigh more. This calculator uses the Hadlock in utero weight chart, the type of population chart recommended by the Society for Maternal-Fetal Medicine (SMFM)¹ ².
What Fetal Weight Percentile Measures
It measures size for the week, not health. A scan cannot weigh a baby directly. The EFW is worked out from measurements of the head, belly and thigh bone, so it is an estimate with a margin of error³. A percentile is one piece of information about growth, not a diagnosis.
The Formula
Median weight (g) = e^(0.578 + 0.332 x GA - 0.00354 x GA²)
z = (EFW ÷ median weight - 1) ÷ 0.127
Percentile = share of a normal bell curve that falls below z
EFW from all four measurements (cm): log10 EFW = 1.3596 - 0.00386 x AC x FL + 0.0064 x HC + 0.00061 x BPD x AC + 0.0424 x AC + 0.174 x FL
GA is weeks pregnant as a decimal. The median curve comes from 392 women scanned in the United States, with a spread of 12.7% of the median at every week¹. The EFW formula is from Hadlock's 1985 study⁴.
How This Calculator Computes Your Result
Enter the weeks and days from your scan report, then either the EFW (in grams, or pounds and ounces) or the scan measurements in mm or cm. With measurements, the calculator picks the Hadlock formula that matches what you entered: all four (BPD, HC, AC and FL), or three or two of them. AC (abdominal circumference) and FL (femur length) are always needed.
You see your percentile (rounded down, so a result just under the 10th is never shown as the 10th), your band, the EFW in grams and pounds, how far it is from the median, and the weights at the 3rd, 10th, 50th, 90th and 97th percentiles for your exact week and day. You also see how the percentile would move if the scan estimate were 10% too high or too low. The calculator works from 14 weeks 0 days to 41 weeks 6 days, and results after 40 weeks are less certain.
What Your Result Means
Your percentile | What it means |
Below the 3rd | Smaller than about 97 in 100 babies at this week. SMFM describes an estimated weight below the 3rd percentile as a more severe form of fetal growth restriction² |
3rd to 9th | Below the 10th percentile. SMFM uses an estimated weight or abdominal circumference below the 10th percentile to define fetal growth restriction² |
10th to 90th | Within the middle 80 in 100 for this week |
Above the 90th | Larger than about 90 in 100 babies at this week |
For reference, these are the weights at key percentiles on the Hadlock chart, worked out from its published equation¹:
Week | 3rd | 10th | 50th | 90th |
20 | 252 g | 277 g | 331 g | 385 g |
24 | 510 g | 561 g | 670 g | 779 g |
28 | 921 g | 1,013 g | 1,210 g | 1,407 g |
32 | 1,486 g | 1,635 g | 1,953 g | 2,271 g |
34 | 1,810 g | 1,990 g | 2,377 g | 2,764 g |
36 | 2,141 g | 2,356 g | 2,813 g | 3,271 g |
38 | 2,463 g | 2,710 g | 3,236 g | 3,763 g |
40 | 2,755 g | 3,030 g | 3,619 g | 4,208 g |
Small for Dates, Growth Restriction and Your Result
SMFM defines fetal growth restriction (FGR) as an EFW or abdominal circumference below the 10th percentile for the week². This calculator checks the EFW only, not the abdominal circumference. A low percentile does not always mean something is wrong. SMFM's guidance separates growth-restricted babies from those who are constitutionally small, meaning small but healthy². Small for gestational age (SGA) is a different term. It describes a newborn whose birth weight is below the 10th percentile, and babies with FGR on a scan are not always SGA at birth². Your care team reads the percentile together with the rest of the scan and your history.
Why Your Percentile May Differ From Your Scan Report
Charts differ, and clinics are expected to use the chart their country or hospital has chosen³. On the World Health Organization (WHO) chart, the 10th percentile at 40 weeks is 3,084 g⁵, compared with about 3,030 g on Hadlock. The same weight can land on a different percentile.
Hadlock's own table and equation also differ slightly. A 2025 study found that using the printed table instead of the equation would have missed growth restriction in 5.1% of patients⁶. This calculator uses the equation.
Exact dates matter too, so use the weeks and days on your report. Near a cut-off, one gram can change the band: at 28 weeks 0 days, 1,013 g is the 9th percentile and 1,014 g is the 10th. Scan weights also should not be plotted on newborn birth weight charts³.
Estimated Fetal Weight Percentiles by Week
The median estimated fetal weight, with the shaded band from the 10th to the 90th percentile and the 3rd percentile below it.
- 50th percentile
- 3rd percentile
- 10th to 90th percentile
Show data table
| Weeks pregnant | 50th percentile (g) | 3rd percentile (g) | 10th to 90th percentile (g) |
|---|---|---|---|
| 20 weeks | 331 | 252 | 277–385 |
| 24 weeks | 670 | 510 | 561–779 |
| 28 weeks | 1,210 | 921 | 1,013–1,407 |
| 32 weeks | 1,953 | 1,486 | 1,635–2,271 |
| 34 weeks | 2,377 | 1,810 | 1,990–2,764 |
| 36 weeks | 2,813 | 2,141 | 2,356–3,271 |
| 38 weeks | 3,236 | 2,463 | 2,710–3,763 |
| 40 weeks | 3,619 | 2,755 | 3,030–4,208 |
A Note on Accuracy
Ultrasound weight estimates commonly differ from the true weight by 10 to 15%³, so your percentile carries the same error. Formulas that use 3 or 4 measurements are more accurate than those that use only 1 or 2². The Hadlock chart was built from mostly white, middle-class women in the United States¹. This calculator is for single pregnancies only. This result is an estimate, not a diagnosis.
Frequently Asked Questions
What is a normal fetal weight percentile?
Anywhere from the 10th to the 90th percentile is the middle 80 in 100 for the week. Below the 10th percentile is the cut-off SMFM uses to define fetal growth restriction².
What does it mean if my baby is below the 10th percentile?
It means the estimated weight is smaller than at least 90 in 100 babies at that week on the chart used. Some of these babies are small but healthy, so your care team looks at the full scan before deciding what it means².
How accurate is ultrasound fetal weight?
Ultrasound estimates of fetal weight commonly have errors of 10 to 15%³. A single scan is a snapshot, which is why this calculator shows how the percentile would change if the estimate were 10% off.
Why is my calculator result different from my ultrasound report?
Your report may use a different chart, such as WHO or INTERGROWTH-21st³, or Hadlock's printed table rather than its equation⁶. Small differences in weeks and days can also shift the percentile.
Sources
In Utero Analysis of Fetal Growth: A Sonographic Weight Standard, Hadlock et al., 1991
Diagnosis and Management of Fetal Growth Restriction (SMFM Consult Series #52), Society for Maternal-Fetal Medicine, 2020
Ultrasound Assessment of Fetal Biometry and Growth (ISUOG Practice Guidelines), Salomon et al., 2019
Estimation of Fetal Weight With the Use of Head, Body, and Femur Measurements, Hadlock et al., 1985
The World Health Organization Fetal Growth Charts, Kiserud et al., 2017
Revisiting the Hadlock 1991 Population Reference for Estimated Fetal Weight, Roberts et al., 2025