Pregnancy Percentile Calculator (Fetal Weight)
Estimate fetal growth percentile using gestational age and estimated fetal weight (EFW) from ultrasound.
What does a pregnancy percentile mean?
A fetal growth percentile compares your baby’s estimated size to a reference population at the same gestational age. For example, a 50th percentile estimate means the weight is near the middle of the reference range. A 10th percentile means the estimate is larger than about 10% of babies and smaller than about 90% at that week.
Percentiles are commonly discussed after ultrasound measurements such as head circumference, abdominal circumference, femur length, and estimated fetal weight are calculated by your provider.
How to use this percentile calculator pregnancy tool
- Enter your current gestational age in weeks (20 to 42 weeks).
- Enter estimated fetal weight from your ultrasound report.
- Select grams or ounces.
- Click Calculate Percentile to get percentile and growth range context.
What result should you focus on?
The most useful part is not one isolated number, but the trend over time. Providers usually compare serial measurements to see whether growth remains steady, accelerates, or slows.
How this calculator works
This tool uses a smoothed week-by-week reference mean for fetal weight and estimates a percentile with a normal-distribution approximation. It then shows your baby’s relative position and a quick interpretation:
- < 10th percentile: lower than expected range (possible SGA pattern)
- 10th to 90th percentile: common range
- > 90th percentile: higher than expected range (possible LGA pattern)
| Gestational Week | Reference Median Weight (g) |
|---|---|
| 24 | 660 g |
| 28 | 1130 g |
| 32 | 1780 g |
| 36 | 2530 g |
| 40 | 3260 g |
How to interpret your percentile carefully
Lower percentile values
Lower values can be completely normal in constitutionally smaller babies. But if growth drops significantly across visits, your care team may evaluate placental function, maternal blood pressure, nutrition, and other factors.
Higher percentile values
Higher values can also be normal. In some pregnancies, persistent high percentiles may trigger additional screening for factors like gestational diabetes or delivery planning considerations.
Why one scan is not the whole story
Ultrasound has natural measurement variability. A single estimate can differ from actual weight, especially late in pregnancy. This is why repeated measurements and clinical context matter much more than one data point.
Factors that can affect fetal growth percentile
- Parental body size and genetics
- Placental health and blood flow
- Maternal conditions (e.g., hypertension, diabetes)
- Smoking, nutrition, and other environmental factors
- Singleton vs. twin pregnancy growth patterns
When to contact your provider
If your report mentions terms like fetal growth restriction (FGR), small for gestational age (SGA), large for gestational age (LGA), or if your percentile changes rapidly between visits, discuss it promptly with your clinician. They may recommend follow-up ultrasound, Doppler studies, or closer prenatal monitoring.
FAQ
Is this the same as a due date calculator?
No. A due date calculator estimates estimated date of delivery, while this tool estimates fetal weight percentile at a given gestational age.
Can I use this for twins?
This calculator is intended for singleton reference patterns. Twin pregnancies use different growth charts and clinical interpretation.
Does a low percentile automatically mean a problem?
Not always. Many healthy babies are naturally smaller. Clinical diagnosis depends on trend, Doppler data, amniotic fluid, and overall maternal-fetal assessment.