Fetal Weight Percentile Calculator
Enter gestational age and your ultrasound estimated fetal weight (EFW) to estimate percentile position versus a reference growth curve.
| Week | Typical 50th percentile weight |
|---|---|
| 20 | 331 g |
| 28 | 1210 g |
| 32 | 1944 g |
| 36 | 2786 g |
| 40 | 3619 g |
Educational use only. This tool does not diagnose fetal growth restriction or macrosomia. Always review results with your OB/GYN or midwife.
What is a pregnancy percentile?
A pregnancy percentile compares your baby's estimated measurement to a reference population at the same gestational age. In this calculator, we focus on estimated fetal weight percentile. If your result is the 60th percentile, it means the estimate is higher than about 60% of babies at that week and lower than about 40%.
Percentiles are a way to track growth trends over time, not a score of “good” or “bad.” Healthy babies can naturally be smaller or larger.
How this pregnancy percentile calculator works
Inputs used
- Gestational age (weeks): usually based on early dating scan or last menstrual period adjusted by your clinician.
- Estimated fetal weight: from ultrasound biometry formulas (for example, using head, abdomen, and femur measurements).
Calculation method
The tool uses a week-by-week reference median fetal weight and linearly interpolates between weeks for decimal ages (for example, 31.6 weeks). It then estimates percentile using a normal approximation around the median for that week.
This is useful for education and quick screening. Clinical teams may use specific standards (Hadlock, INTERGROWTH-21st, customized charts, or local population curves), so results can vary slightly across systems.
How to interpret your result
- Below 10th percentile: often flagged for closer follow-up, especially if trend stays low or Doppler studies are abnormal.
- 10th to 90th percentile: generally considered within expected range for gestational age.
- Above 90th percentile: may prompt monitoring for large-for-gestational-age patterns and delivery planning.
One single percentile rarely tells the whole story. Your clinician looks at trend over time, placenta function, fluid levels, maternal health, and other ultrasound findings.
Why percentile can change between scans
It is normal for percentile to move up or down somewhat from one visit to the next. Reasons include:
- Natural growth variation between babies
- Ultrasound measurement uncertainty (EFW is an estimate, not an exact weight)
- Different scan operators or fetal position during imaging
- Real growth acceleration or deceleration
Practical tips for more useful tracking
1) Use consistent timing
Comparing scans done at similar intervals (for example, every 2–4 weeks if advised) makes trends more meaningful.
2) Bring your prior reports
Trend interpretation is strongest when your care team can compare measurements side-by-side.
3) Don’t self-diagnose from one number
Percentile is only one part of prenatal care. Discuss the full clinical picture before drawing conclusions.
Common questions
Is 50th percentile “best”?
No. 50th percentile is simply the median. Many healthy babies are naturally below or above this point.
Can a baby at the 15th percentile still be healthy?
Yes. Context matters. If Dopplers, fluid, and interval growth are reassuring, a lower percentile can still be normal for that baby.
What if the result is very low or very high?
Use the result as a prompt to contact your prenatal team, not as a final diagnosis. They may recommend repeat growth scan, Doppler assessment, or additional monitoring.
Important safety note
This pregnancy percentile calculator is educational and should not replace medical advice. If you have concerning symptoms such as decreased fetal movement, vaginal bleeding, severe headache, abdominal pain, or fluid leakage, seek urgent medical care immediately.