Standard Formulas for Estimating Fetal Birth Weight Using Ultrasound
The Hadlock formula incorporating head circumference (HC), abdominal circumference (AC), and femur length (FL) is the most accurate method for estimating fetal weight across all gestational ages and should be used as the standard approach. 1
Primary Recommendation: Hadlock Formula
The 1985 Hadlock formula using HC, AC, and FL measurements provides:
- The highest accuracy with 80% of estimates within 10% of actual birth weight 1
- A correlation coefficient of 0.959 with actual birth weight 1
- The lowest Euclidean distance and highest proportion of acceptable error across all weight ranges 1
This formula outperforms all 70 other published models for estimated fetal weight (EFW), including newer formulas like INTERGROWTH. 1
Required Ultrasound Measurements
Fetal biometry for weight estimation should include 2:
- Biparietal diameter (BPD)
- Head circumference (HC)
- Abdominal circumference (AC)
- Femur length (FL)
These measurements are combined using regression equations to generate the estimated fetal weight 3.
Alternative Formulas for Specific Clinical Scenarios
For Very Low Birth Weight Fetuses (<1500g)
- The Hadlock formulas remain most accurate even for very small fetuses 4
- Specific formulas designed for fetuses <1500g do not substantially improve accuracy over Hadlock 4
- The Scott formula showed the narrowest limits of agreement in this population but was not superior to Hadlock 4
For Preterm Fetuses (22-34 weeks)
- Hadlock formula significantly outperforms INTERGROWTH in preterm populations 5
- Mean percentage error for Hadlock: -0.7% vs INTERGROWTH: -3.5% (p<0.001) 5
- 68.7% of estimates within ±10% of birthweight using Hadlock vs 57.8% using INTERGROWTH 5
For Macrosomic Fetuses
- Merz's regression formula shows best performance for macrosomic newborns 6
- Gender-specific Schild formula performs well for infants 2500-3999g 6
Critical Clinical Context
Importance of Accurate Dating
Accurate pregnancy dating using first-trimester crown-rump length is essential for interpreting fetal weight estimates, as misdating leads to misclassification of growth abnormalities. 7, 3
Clinical Thresholds for Action
- Fetal growth restriction (FGR): EFW or AC <10th percentile for gestational age 3
- Severe FGR: EFW <3rd percentile, associated with increased adverse outcomes regardless of Doppler findings 7, 3
- Small for gestational age: EFW <10th percentile 2
Measurement Standards
Birth weight should be collected within 24 hours of birth using a calibrated electronic scale with 10-g resolution 2.
Common Pitfalls to Avoid
Examiner experience significantly impacts accuracy - skilled sonographers achieve much higher correlation coefficients across all formulas 6. This emphasizes the importance of:
- Proper training in obtaining biometric measurements
- Quality assurance programs for ultrasound measurements
- Recognition that formula accuracy depends on measurement precision
Do not use customized growth standards - the American College of Obstetricians and Gynecologists recommends population-based references rather than customized standards when determining fetal weight percentiles 7, 3.