Fetal Weight Calculator (Hadlock)
Estimate fetal weight from ultrasound biometry using the Hadlock formula.
Important Clinical Disclaimer:
Fetal Weight Calculator is intended for use by qualified healthcare professionals only.
It is for educational and informational purposes and should not replace clinical judgement.
Always interpret results alongside the full clinical picture and follow your local guidelines and protocols.
About Fetal Weight Estimation
Background
The Hadlock formulas were developed by Dr Frank Hadlock and colleagues in the 1980s and remain among the most widely used methods for estimating fetal weight from ultrasound measurements. Multiple formulas exist using different combinations of measurements.
Hadlock Formulas
| Formula | Parameters | Typical Error |
|---|---|---|
| Hadlock IV | BPD + HC + AC + FL | ~7.5% |
| Hadlock III | AC + FL | ~8% |
| Hadlock I | AC only | ~15% |
Measurement Technique
- BPD: Measured at the level of the thalami and cavum septum pellucidum, from outer edge to inner edge of the skull
- HC: Ellipse around the outer edge of the skull at the BPD level
- AC: Circular measurement at the level of the stomach bubble and umbilical vein (J-shaped portal sinus)
- FL: Length of the ossified femoral diaphysis, excluding epiphyses
Accuracy and Limitations
- 85% of estimates are within 15% of actual birth weight
- Accuracy decreases at extremes of fetal weight
- Oligohydramnios may affect AC measurement
- Fetal position can affect measurement quality
- All formulas tend to overestimate small fetuses and underestimate large fetuses
UK Fetal Weight Centiles (Intergrowth-21st)
| Gestation | 3rd | 10th | 50th | 90th | 97th |
|---|---|---|---|---|---|
| 24 weeks | 496g | 548g | 665g | 806g | 888g |
| 28 weeks | 817g | 908g | 1113g | 1364g | 1512g |
| 32 weeks | 1294g | 1450g | 1805g | 2246g | 2508g |
| 36 weeks | 1965g | 2214g | 2790g | 3517g | 3954g |
| 40 weeks | 2621g | 2966g | 3769g | 4790g | 5411g |
Reference: INTERGROWTH-21st international standards. Individual customised centiles (e.g., GROW) may be more appropriate for clinical management.
Frequently Asked Questions
Estimated fetal weight (EFW) is a calculation of the baby's weight in the womb based on ultrasound measurements. The Hadlock formula uses four measurements - biparietal diameter, head circumference, abdominal circumference, and femur length - to estimate weight with an accuracy of approximately 85% within 15% of actual birth weight.
The Hadlock formula is one of the most widely used methods for estimating fetal weight. The Hadlock IV formula uses four parameters (BPD, HC, AC, FL) and is: Log10(EFW) = 1.3596 + 0.0064(HC) + 0.0424(AC) + 0.174(FL) + 0.00061(BPD)(AC) - 0.00386(AC)(FL). Alternative Hadlock formulas use fewer parameters.
Ultrasound fetal weight estimation using the Hadlock formula is accurate to within 15% of actual birth weight approximately 85% of the time. Accuracy may be reduced in very large or very small babies, with oligohydramnios, or at extreme gestational ages.
Normal fetal weight varies significantly with gestational age. A weight between the 10th and 90th centile for gestational age is generally considered normal. Below the 10th centile may indicate small for gestational age (SGA) or fetal growth restriction (FGR), while above the 90th centile suggests large for gestational age (LGA).
References
- Hadlock FP, Harrist RB, Sharman RS, et al. Estimation of fetal weight with the use of head, body, and femur measurements. Am J Obstet Gynecol. 1985;151(3):333-337. PMID: 3881966
- Papageorghiou AT, et al. International standards for fetal growth (INTERGROWTH-21st). Lancet. 2014;384(9946):869-879. PMID: 25209488
- RCOG Green-top Guideline No. 31. The Investigation and Management of the Small-for-Gestational-Age Fetus. 2014. RCOG GTG 31
- Dudley NJ. A systematic review of the ultrasound estimation of fetal weight. Ultrasound Obstet Gynecol. 2005;25(1):80-89. PMID: 15505877
Weight Centile Categories
<3rd centile: Severely SGA
High risk, needs urgent assessment
3rd-10th centile: SGA
Further assessment recommended
10th-90th: Normal
Appropriate for gestational age
90th-97th: LGA
Monitor for macrosomia
>97th: Severely LGA
Consider gestational diabetes screening
Measurement Tips
BPD
- Plane: thalami and cavum septum pellucidum visible
- Measure outer-to-inner skull table
HC
- Same plane as BPD
- Ellipse around outer skull edge
AC
- Stomach bubble visible
- Portal vein J-shape
- Circular cross-section (not oblique)
FL
- Both ends clearly visible
- Exclude epiphyses
- Femur as horizontal as possible
Clinical Significance
Small for Gestational Age (SGA)
- EFW <10th centile
- May indicate fetal growth restriction
- Check umbilical artery Doppler
- Assess for causes (placental, fetal, maternal)
Large for Gestational Age (LGA)
- EFW >90th centile
- Screen for gestational diabetes
- Consider delivery planning
- Monitor for shoulder dystocia risk
Disclaimer
This calculator is provided for educational and informational purposes only. It is not intended to be a substitute for professional medical advice, diagnosis, or treatment.
- Always seek the advice of a qualified healthcare provider with any questions regarding a medical condition.
- Never disregard professional medical advice or delay seeking it because of information from this tool.
- Clinical decision-making should always incorporate the full clinical context, patient preferences, and local protocols.
- The creators of this tool accept no liability for decisions made based on its output.
If you are a patient: please discuss any results with your healthcare provider. This tool is designed for use by medical professionals and may not be appropriate for self-assessment.