How to Interpret Gestational Age Using Hadlock Formulas
Use the Hadlock formula incorporating head circumference (HC), femur length (FL), and abdominal circumference (AC) to calculate estimated fetal weight (EFW), then plot this value on reference charts developed specifically using Hadlock-derived EFW to determine gestational age percentiles. 1
Which Hadlock Formula to Use
The Hadlock formula using HC, FL, and AC (without biparietal diameter/BPD) is the recommended standard for fetal weight estimation 1. Adding BPD does not increase accuracy and introduces unnecessary population variation 1. This three-parameter formula has demonstrated superior accuracy compared to alternatives like the Intergrowth 21 equation 1.
Critical Technical Requirements for Accurate Interpretation
Measurement Technique Must Match Reference Standards
You must use the exact same measurement methodology that was used to create your reference curves 1. This is a common pitfall that leads to misinterpretation:
- For Hadlock formula: Use the ellipse method for both HC (transthalamic section) and AC 1
- Head circumference: Measure at the transthalamic plane (not transventricular) when using Hadlock 1
- Different measurement techniques (ellipse vs. two-diameter methods) will yield different results and cannot be interchanged 1
Plotting on Appropriate Growth Charts
Plot your Hadlock-derived EFW only on reference charts that were developed using the Hadlock equation 1. Using mismatched charts and formulas creates systematic errors in gestational age interpretation.
Understanding the Limitations
Recognize Inherent Imprecision
The Hadlock formula has an inherent margin of error of ±10%, with greater errors at:
- Extremes of fetal weight
- Extremes of gestational age 1
Image quality significantly affects accuracy—poor quality biometric images reduce EFW precision 1.
Avoid Formula Switching During Serial Monitoring
Changing formulas during growth monitoring creates artifactual changes in growth trajectory 1. For example:
- EFW calculated with HC + AC + FL differs from AC + FL alone depending on fetal proportions
- If you must switch formulas (e.g., unable to obtain HC in late gestation), explicitly account for this when comparing to previous assessments 1
Quality Assurance Requirements
Those interpreting Hadlock measurements must ensure:
- Appropriate training in fetal biometry assessment 1
- Standardized planes of acquisition and caliper placement 1
- Quality control of images and measurements 1
- Regular audit to monitor that the proportion of fetuses <3rd percentile matches expected population norms 1
Clinical Context for Interpretation
When using Hadlock for gestational age assessment:
- Define fetal growth restriction as EFW <10th percentile for gestational age 2
- Use population-based fetal growth references (such as Hadlock) rather than customized charts 2
- Abdominal circumference alone is comparable to EFW for predicting small-for-gestational-age, though EFW remains superior 1
Common Pitfall to Avoid
The single most critical error is using measurement techniques from one methodology while plotting on charts derived from another. This creates systematic bias that invalidates gestational age interpretation. Always verify that your ultrasound machine's measurement protocol matches the reference chart you're using for interpretation 1.