Estimate Fetal Weight Percentile
Enter gestational age and estimated fetal weight (EFW) from ultrasound to estimate percentile ranking.
This tool is educational and does not diagnose growth restriction or macrosomia. Always confirm interpretation with your obstetric clinician.
How this fetal weight percentile calculator works
This calculator estimates where your baby's ultrasound-estimated weight falls compared with other pregnancies at the same gestational age. The result is a percentile:
- 50th percentile means exactly average for that gestational age.
- Below 10th percentile may suggest smaller-than-expected growth and often prompts closer follow-up.
- Above 90th percentile may suggest larger-than-expected growth.
Because ultrasound is an estimate (not a scale measurement), individual values can be off by around 10-15% in either direction, especially later in pregnancy.
Inputs you need
1) Gestational age
Use your dating scan or clinician-confirmed due date. Enter completed weeks and extra days. Example: 32 weeks and 4 days should be entered as 32 and 4.
2) Estimated fetal weight (EFW)
Use the EFW in grams from your ultrasound report. Sonographers compute this from measurements such as abdominal circumference, biparietal diameter, and femur length.
How to interpret the percentile
- 3rd to 97th percentile: often considered within broad expected range.
- < 10th percentile: your care team may evaluate placental function, Dopplers, and interval growth.
- > 90th percentile: your care team may monitor for maternal diabetes, delivery planning factors, and shoulder dystocia risk.
One value alone is less important than the trend over time. Serial ultrasounds can show whether growth is stable, accelerating, or slowing.
Reference growth anchors (median weights)
The calculator uses a smoothed, week-by-week fetal growth reference to estimate median weight and spread at each gestational age.
| Gestational Age | Median EFW (g) | Typical Clinical Context |
|---|---|---|
| 20 weeks | 331 g | Mid-pregnancy anatomy period |
| 24 weeks | 670 g | Late second trimester growth checks begin to matter more |
| 28 weeks | 1,210 g | Third trimester trend tracking |
| 32 weeks | 1,953 g | Common window for repeat growth scan |
| 36 weeks | 2,813 g | Birth-planning discussions increase |
| 40 weeks | 3,619 g | Term comparison point |
Important limitations
- Ultrasound EFW has built-in error and is not exact birth weight.
- Different growth charts (Hadlock, INTERGROWTH-21st, customized charts) can yield different percentiles.
- Maternal conditions, fetal sex, ethnicity, altitude, and parity can influence expected fetal size.
- Clinical decisions should never be based on one online estimate alone.
When to contact your prenatal team
Speak with your obstetric provider promptly if your report mentions fetal growth restriction (FGR), abnormal Dopplers, oligohydramnios, polyhydramnios, or rapid interval growth changes. Percentiles are useful screening markers, but the complete picture includes fluid, placenta, blood flow, maternal history, and fetal wellbeing testing.
FAQ
Is a low percentile always dangerous?
No. Some babies are constitutionally small and healthy. Risk rises when low percentile is paired with abnormal Dopplers, reduced growth velocity, or other warning signs.
Can percentile change quickly?
Yes, especially if dating changes, measurement variability occurs, or growth trajectory shifts between scans.
Should I compare this with birth-weight percentiles?
Not directly. Prenatal EFW percentile and postnatal birth-weight percentile use different contexts and methods.