Fetal Weight Estimator
Estimate fetal weight using the Hadlock formula based on ultrasound biometry (BPD, HC, AC, FL). The Hadlock formula is the most widely used method with accuracy of ±15% of actual birth weight.
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Key values and context for this tool: • Hadlock Accuracy: ±15% • Parameters: BPD, HC, AC, FL • Normal Range: 10th-90th %ile
Ready to run the numbers?
Why: Calculate estimated fetal weight from ultrasound measurements using Hadlock formula.
How: Enter your values in the calculator fields below, keep units consistent, then run the calculation to see results and any step-by-step work shown on this page.
Run the calculator when you are ready.
Sample Scenarios
📅 20 Weeks
Mid-pregnancy anatomy scan → ~300g
📅 28 Weeks
Third trimester growth check → ~1050g
📅 32 Weeks
Growth scan - normal → ~1750g
📅 36 Weeks
Near term assessment → ~2725g
📅 40 Weeks
Full term assessment → ~3735g
📏 AC Only
Minimal input (Hadlock 1) → basic estimate
Enter Ultrasound Measurements
For informational purposes only — not medical advice. Consult a healthcare professional before acting on results.
Understanding Estimated Fetal Weight
Estimated Fetal Weight (EFW) is calculated from ultrasound measurements using validated formulas. All estimates have approximately ±15% margin of error.
BPD
Biparietal Diameter — Head width
HC
Head Circumference
AC
Abdominal Circumference — Most predictive
FL
Femur Length
Growth Categories by Percentile
SGA (<10th percentile)
Small for Gestational Age — May need additional monitoring
AGA (10th-90th percentile)
Appropriate for Gestational Age — Normal growth range
LGA (>90th percentile)
Large for Gestational Age — Screen for gestational diabetes
When is EFW Measured?
18-22 Weeks: Anatomy Scan
Detailed evaluation of fetal anatomy and baseline growth
28-32 Weeks: Growth Check
Third trimester assessment, especially if high-risk
36+ Weeks: Delivery Planning
Assessment for delivery considerations
Hadlock Formula Accuracy
Hadlock 4-parameter formula (BPD, HC, AC, FL) is the most widely used method with ±15% accuracy. More parameters generally improve precision.
Factors Affecting EFW Accuracy
- Operator skill and experience
- Fetal position and movement
- Maternal BMI (obesity reduces accuracy)
- Gestational age (late third trimester less accurate)
Clinical Implications
EFW guides prenatal care, delivery planning, and screens for growth restriction (IUGR) or macrosomia. Always discuss results with your healthcare provider.
Normal Weight Ranges by Gestational Age
Average (50th percentile) fetal weights: 20w ~300g, 28w ~1050g, 32w ~1750g, 36w ~2725g, 40w ~3735g. The 10th-90th percentile range is considered normal.
When to Seek Further Evaluation
Consult your provider if EFW is below 10th or above 90th percentile, or if growth velocity changes significantly between scans.
Disclaimer
This calculator provides estimates only. All ultrasound weight estimates have approximately ±15% margin of error. This tool does not replace professional medical advice.
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