Fetal Weight Percentile Calculator

About the Fetal Weight Percentile Calculator

The Fetal Weight Percentile Calculator estimates fetal weight from standard ultrasound biometry using the Hadlock formula. It combines biparietal diameter, head circumference, abdominal circumference, and femur length, then compares the result with a gestational-age reference range.

That gives you an estimated fetal weight, an approximate percentile, and a basic classification such as small for gestational age, appropriate for gestational age, or large for gestational age. The percentile view is useful because it puts a single weight estimate into the context of expected growth at the same gestational age.

The page is aimed at quick growth screening, not at replacing a full obstetric assessment or serial trend review.

When This Page Helps

Percentile-based fetal weight estimates help make growth assessment easier to compare across visits and across pregnancies. They are most useful as a screening view that points to possible concern and suggests when closer follow-up or a more complete assessment may be worth considering.

How to Use the Inputs

1. Enter the current gestational age in weeks and days.
2. Enter the BPD measurement from ultrasound in millimeters.
3. Enter the head circumference (HC) in millimeters.
4. Enter the abdominal circumference (AC) in millimeters.
5. Enter the femur length (FL) in millimeters.
6. Review the estimated fetal weight, percentile, and classification.
7. For SGA or LGA fetuses, discuss surveillance and management plans.

Example Calculation

Tips & Best Practices

• AC is the most important single parameter — if only one measurement is available, use AC-based formulas.
• Ensure measurements are obtained according to standardized technique (plane, calipers, averages).
• Growth velocity (interval change) is often more informative than a single percentile.
• Consider Doppler assessment (umbilical artery, middle cerebral artery) for EFW <10th percentile.
• Report uncertainty: an EFW of 2000g means the true weight is likely 1700-2300g.
• Oligohydramnios (AFI <5) alongside SGA significantly increases the concern for true FGR.

Hadlock Formula Variants

Multiple Hadlock equations exist using different parameter combinations. The 4-parameter formula (BPD, HC, AC, FL) provides the best overall accuracy. When BPD is unreliable (dolichocephaly, engaged head), the 3-parameter HC+AC+FL formula is preferred. Select the appropriate formula based on measurement quality and fetal position.

Fetal Growth Restriction Management

Early-onset FGR (<32 weeks) is often associated with placental insufficiency and abnormal uterine artery Doppler waveforms. Management includes serial ultrasound, Doppler surveillance (umbilical artery, MCA, ductus venosus), and timed delivery. Late-onset FGR (≥32 weeks) may have normal Dopplers and requires cerebro-placental ratio (CPR) assessment for detection.

Emerging Technology

AI-assisted ultrasound biometry is showing promise for reducing inter-operator variability. Magnetic resonance imaging provides more accurate fetal weight estimation in obese patients and at the extremes of fetal weight but is not widely available for routine prenatal care.

Frequently Asked Questions

• How accurate is ultrasound EFW?

  The Hadlock formula has a systematic error of ±10-15% (95% CI). Accuracy decreases at extremes — very small and very large fetuses have greater prediction error. Adjacent measurements (e.g., 1800g ± 250g) represent a clinically significant range. Serial growth trend assessment is more informative than a single EFW.

• Which biometry parameters are most important?

  AC is the single most predictive measurement for EFW (r² ~0.75). Combining AC with FL gives nearly as good accuracy as the 4-parameter Hadlock formula. HC and BPD add incremental accuracy but are affected by head shape variability (dolichocephaly).

• How often should growth scans be performed?

  For normal pregnancies, growth assessment at 28-32 weeks is standard. For suspected FGR, serial scans every 2-3 weeks monitor velocity. More frequent scanning (<2 weeks) may not show detectable growth changes due to measurement variability.

• What is the difference between SGA and FGR?

  SGA (small for gestational age) is a statistical definition: EFW <10th percentile. FGR (fetal growth restriction) is a pathologic diagnosis suggesting the fetus has not reached its growth potential. Some SGA fetuses are constitutionally small but healthy; some AGA fetuses may be growth restricted if their genetically expected weight is higher.

• When does LGA require intervention?

  LGA (>90th percentile) should prompt screening for gestational diabetes (OGTT if not done). EFW >4500g in diabetic mothers or >5000g in non-diabetic mothers is an indication to discuss planned cesarean due to shoulder dystocia risk.

• Should customized growth charts be used?

  Sometimes. Different services use different fetal-growth standards or customized charts, and that can change percentile classification. This page is best used as a quick screening estimate rather than as a substitute for the exact growth standard used in your obstetric service.