Hadlock EFW Calculator — Fetal Biometry Suite
Clinical-grade estimated fetal weight from ultrasound measurements — Hadlock 1985 formula.
Enter BPD, AC and FL (all required). Add HC to automatically switch to the more accurate 4-parameter formula. Add GA at scan to see the growth percentile classification (SGA / AGA / LGA). All measurements in millimetres (mm).
Need EDD, gestational age, Bishop score, VBAC or GDM screening too?
This EFW calculator is part of the full ObsCalc obstetric suite — the digital OB wheel.
Fetal Weight Percentiles by Gestational Age
Reference median (50th percentile) fetal weights by gestational week, based on Hadlock/Alexander growth standards. Use this alongside the calculator above — an EFW below the 10th percentile line or above the 90th percentile line for a given week warrants closer surveillance.
| GA (weeks) | 10th %ile (approx.) | 50th %ile (median) | 90th %ile (approx.) |
|---|---|---|---|
| 20w | 240 g | 300 g | 360 g |
| 22w | 350 g | 430 g | 510 g |
| 24w | 480 g | 600 g | 720 g |
| 26w | 620 g | 760 g | 930 g |
| 28w | 820 g | 1005 g | 1230 g |
| 30w | 1080 g | 1319 g | 1620 g |
| 32w | 1400 g | 1702 g | 2090 g |
| 34w | 1780 g | 2146 g | 2620 g |
| 36w | 2180 g | 2622 g | 3160 g |
| 38w | 2570 g | 3083 g | 3700 g |
| 40w | 2880 g | 3462 g | 4140 g |
Values are indicative averages for reference only; local population-specific charts (e.g. INTERGROWTH-21st or Indian-specific standards) may be preferred for individual clinical decisions.
Hadlock EFW — Clinical Guide
What is the Hadlock EFW Formula?
The Hadlock formula (Hadlock et al., 1985) is the most widely used method for estimating fetal weight from ultrasound biometry. It uses a regression equation on log10(estimated weight) built from combinations of Biparietal Diameter (BPD), Head Circumference (HC), Abdominal Circumference (AC), and Femur Length (FL). The 4-parameter formula (BPD+HC+AC+FL) is generally preferred for accuracy; the 3-parameter formula (BPD+AC+FL) is used when HC cannot be reliably measured, such as with a deeply engaged head near term.
How to Use This EFW Calculator
Measure BPD, AC and FL on the same ultrasound scan and enter them in millimetres. If HC was also measured, enter it too — the calculator automatically switches to the more accurate 4-parameter formula whenever HC is provided. Optionally enter the gestational age at the time of scan to see where the estimate falls on the growth percentile curve (SGA, AGA, or LGA).
Interpreting the Result — SGA, AGA and LGA
An EFW below the 10th percentile for gestational age is classified as small for gestational age (SGA); below the 3rd percentile is often considered severe SGA and raises concern for fetal growth restriction (FGR/IUGR), particularly if accompanied by abnormal umbilical artery Doppler. An EFW above the 90th percentile is classified as large for gestational age (LGA), and an EFW ≥4000–4500g at term is generally used to flag suspected macrosomia — relevant to delivery planning, especially with gestational diabetes.
Accuracy and Limitations
Hadlock EFW carries an inherent variability of approximately ±15%; the true birth weight typically falls within this range in roughly 95% of pregnancies. Accuracy decreases at the extremes of fetal weight, with maternal obesity, oligohydramnios, or poor acoustic windows, and closer to term. EFW is best used for trend monitoring across serial scans rather than as a standalone determinant of mode or timing of delivery.
3-Parameter vs 4-Parameter Formula
| Feature | 3-Parameter (BPD+AC+FL) | 4-Parameter (BPD+HC+AC+FL) |
|---|---|---|
| Measurements needed | BPD, AC, FL | BPD, HC, AC, FL |
| When used | HC not obtainable (e.g. deeply engaged head) | Preferred whenever HC is available |
| Accuracy | Good | Slightly improved |
| Used by this calculator | Auto-selected if HC left blank | Auto-selected if HC entered |
Frequently Asked Questions
What is the Hadlock formula for estimated fetal weight?
The Hadlock 1985 formula estimates fetal weight from ultrasound biometry using a regression equation on log10(weight). The 4-parameter formula uses BPD, HC, AC and FL; the 3-parameter formula (BPD, AC, FL) is used when HC cannot be measured. It remains the most widely used EFW formula in obstetric practice, including in India.
How accurate is Hadlock EFW?
Hadlock EFW has an inherent variability of about ±15%, meaning the true birth weight typically falls within 15% of the estimate in roughly 95% of cases. Accuracy is lower at extremes of fetal weight and with maternal obesity or oligohydramnios affecting image quality.
What is the difference between the 3-parameter and 4-parameter Hadlock formula?
The 4-parameter formula (BPD+HC+AC+FL) is generally preferred as it slightly improves accuracy by including head circumference. The 3-parameter formula (BPD+AC+FL) is used when HC is technically difficult to obtain, such as with a deeply engaged fetal head, and gives comparable results.
What EFW percentile indicates SGA or IUGR?
An EFW below the 10th percentile for gestational age is classified as small for gestational age (SGA), and below the 3rd percentile is often considered severe SGA or suggestive of fetal growth restriction (FGR/IUGR), especially with abnormal Doppler findings. Serial growth scans and Doppler studies are recommended for confirmation.
What EFW percentile indicates macrosomia or LGA?
An EFW above the 90th percentile for gestational age is classified as large for gestational age (LGA). An EFW at or above 4000–4500g at term is generally used to flag suspected macrosomia, which raises concern for shoulder dystocia and may influence delivery planning, especially in women with gestational diabetes.
Which measurements do I need for the Hadlock EFW calculator?
You need Biparietal Diameter (BPD), Abdominal Circumference (AC), and Femur Length (FL) in millimetres, all obtained on the same ultrasound scan. Head Circumference (HC) is optional but improves accuracy when included. Entering gestational age also allows the calculator to show a growth percentile.