Estimate fetal growth percentile
Use gestational age and estimated fetal weight (EFW) from ultrasound to get an approximate percentile.
Important: This tool is for education only and does not diagnose fetal growth restriction or macrosomia. Always review results with your OB-GYN or midwife.
What this fetal percentile calculator does
A fetal percentile compares your baby's estimated fetal weight (EFW) to babies at the same gestational age. For example, a 50th percentile means the estimated weight is near the middle of the reference population, while the 10th percentile means about 10% of babies weigh less and 90% weigh more at that same week of pregnancy.
This calculator gives a quick estimate based on commonly used fetal growth reference behavior and an interpolated median by gestational week. It is most useful for tracking trends over time rather than relying on one isolated number.
How to use it
- Enter gestational age in completed weeks and extra days.
- Enter the estimated fetal weight from your latest ultrasound report.
- Select the unit (grams, ounces, or pounds).
- Click Calculate Percentile.
You will receive:
- Estimated percentile
- Estimated median weight for that age
- Difference from median (higher or lower)
- Simple category label (SGA / AGA / LGA)
How to interpret percentile ranges
Common clinical cut points
- Below 10th percentile: often labeled small for gestational age (SGA)
- 10th to 90th percentile: typically appropriate for gestational age (AGA)
- Above 90th percentile: often labeled large for gestational age (LGA)
Percentiles are screening tools, not final diagnoses. Ultrasound measurements, fetal position, and machine differences can all affect EFW estimates. Your care team will combine this with amniotic fluid, Dopplers, maternal health, and interval growth.
Reference median weights used by this tool (approximate)
| Week | Median EFW (g) | Week | Median EFW (g) |
|---|---|---|---|
| 20 | 390 | 31 | 1670 |
| 22 | 540 | 32 | 1820 |
| 24 | 730 | 34 | 2140 |
| 26 | 960 | 36 | 2480 |
| 28 | 1230 | 38 | 2840 |
| 30 | 1520 | 40 | 3200 |
Medians are interpolated across 14-42 weeks for smooth estimates. Real clinical software may use Hadlock, INTERGROWTH-21st, WHO, or local population curves.
Why trend matters more than a single value
A one-time percentile can be noisy. Serial ultrasounds performed at proper intervals are usually more informative. A baby at the 20th percentile that follows a stable growth path may be completely healthy, while a baby dropping rapidly from the 60th to the 15th percentile may need closer evaluation.
When to call your prenatal team promptly
- Percentile appears very low or drops significantly over time
- You notice reduced fetal movement
- You have high blood pressure symptoms, severe swelling, headache, or vision changes
- You have diabetes and growth appears unusually high
- You are unsure how to interpret your ultrasound report
FAQ
Is a low percentile always dangerous?
No. Some babies are constitutionally small and healthy. Risk assessment depends on trends, blood flow studies, fluid volume, and maternal/fetal context.
Can ultrasound be off?
Yes. EFW has a known margin of error. Near term, uncertainty can be substantial, which is why providers avoid decisions based on one number alone.
Does this replace my doctor?
No. Use this as an educational estimate only. Clinical care must come from your licensed prenatal provider.