Fetal Weight Percentile Calculator
Convert ultrasound estimated fetal weight (EFW) into gestational-age percentiles for obstetric decision-making.
Use the best obstetric dating method (LMP + first trimester ultrasound).
Enter the ultrasound EFW in grams.
How to Use This Calculator
Obtain accurate EFW
Use ultrasound biometry (Hadlock, INTERGROWTH) to estimate fetal weight. Note the gestational age at the time of scan.
Enter gestational age
Use the best obstetric dating method (early ultrasound is preferred). Enter age in weeks with decimals if needed.
Interpret percentile
Fetal growth restriction is typically <10th percentile. Macrosomia or LGA is >90th percentile. Combine with clinical findings before deciding interventions.
Formula
1. Interpolate percentile weights (P₁₀, P₅₀, P₉₀) for the gestational age.
Reference values approximate INTERGROWTH-21st fetal growth standards and common obstetric nomograms.
About Fetal Weight Percentiles
Fetal weight percentiles help obstetricians monitor intrauterine growth. Small-for-gestational-age fetuses may require closer surveillance, Doppler velocimetry, or early delivery. Large fetuses raise concerns for shoulder dystocia or cesarean delivery.
Ultrasound EFW has a ±10% error margin. Trends over multiple scans, combined with amniotic fluid and Doppler studies, increase diagnostic accuracy.
Clinical pearls
- Repeat scans every 2–4 weeks to assess growth velocity.
- Consider maternal conditions (preeclampsia, diabetes) when interpreting percentiles.
- Severe growth restriction (<3rd percentile) often triggers earlier intervention.
Frequently Asked Questions
How accurate is ultrasound EFW?
Modern formulas have an error margin of ±10%. Use serial measurements and clinical context for decisions.
What percentile defines fetal growth restriction?
Many guidelines use <10th percentile. <3rd percentile often indicates severe restriction.
Can fetal sex affect EFW?
Male fetuses are slightly heavier on average, but sex-specific charts are not always necessary for screening.
How often should I repeat scans?
Typically every 3–4 weeks for routine monitoring; every 2 weeks or sooner if growth restriction is suspected.
Does a high percentile mean macrosomia?
Percentiles above 90 suggest LGA. Combine with maternal diabetes, gestational age, and estimated birth weight to plan delivery.