Fetal Weight Percentile Calculator
Enter gestational age and estimated fetal weight (EFW) from your ultrasound report to estimate percentile.
This tool provides an educational estimate based on modeled reference growth data (14–42 weeks). It is not a diagnosis.
What does fetal percentile mean?
A fetal percentile compares your baby's estimated size to a large reference population at the same gestational age. For example, if a fetus is at the 60th percentile for weight at 32 weeks, that means about 60% of fetuses at 32 weeks are estimated to weigh less, and about 40% weigh more.
Percentiles are not grades—they are simply position markers on a growth curve. A healthy fetus can be small, average, or large. What matters most is the full clinical picture and growth trend over time.
How this percentile calculator works
This calculator uses:
- Your gestational age in weeks
- Your ultrasound estimated fetal weight (EFW) in grams
- A modeled week-by-week reference curve
- A normal distribution approach to convert z-score into percentile
Behind the scenes, the tool estimates the median weight and expected variability for your selected week, then calculates where your EFW sits relative to that distribution.
How to interpret the result
Typical interpretation bands
- Below 3rd percentile: very low for gestational age; needs close medical review
- 3rd to 10th percentile: below average; may suggest risk of fetal growth restriction in some cases
- 10th to 90th percentile: generally considered the typical range
- 90th to 97th percentile: above average; may suggest larger-than-average growth
- Above 97th percentile: very high for gestational age; clinical follow-up may be needed
Important context
Ultrasound EFW can have meaningful error (often around ±10% or more), especially later in pregnancy. That means percentile values are estimates, not exact measurements.
What can affect fetal percentile?
- Placental function and blood flow
- Maternal health (blood pressure, diabetes, nutrition, thyroid status)
- Genetic factors and parental size
- Singleton vs. multiple pregnancy
- Timing and technique of ultrasound measurements
When to contact your provider
Speak with your obstetric clinician if:
- Your result is below the 10th percentile or above the 90th percentile
- You notice reduced fetal movement
- You were advised to repeat growth scans and are unsure why
- You have concerns about blood pressure, gestational diabetes, or preterm symptoms
Your provider may recommend repeat ultrasound, Doppler studies, NST/BPP monitoring, or additional lab work depending on your situation.
Frequently asked questions
Is one low percentile result always dangerous?
Not necessarily. Some babies are constitutionally small and healthy. The trend across serial ultrasounds is often more informative than a single scan.
Can percentile change over time?
Yes. Percentile can move up or down between scans. Clinicians look for persistent downward crossing or poor interval growth rather than isolated fluctuations.
Is this calculator a replacement for prenatal care?
No. This tool is educational only. Clinical decisions should always be made with your prenatal care team.
Bottom line
A fetal percentile calculator is useful for quick context, but it should be interpreted with ultrasound quality, maternal history, fetal anatomy, amniotic fluid, and Doppler findings. Use this result as a conversation starter with your care team, not as a standalone diagnosis.