About

In obstetric ultrasound, estimating fetal weight (EFW) is crucial for managing delivery, especially in cases of suspected growth restriction or macrosomia. While manual methods like Johnson's formula are useful, ultrasound biometry provides much higher precision by measuring specific skeletal and soft tissue markers.

This calculator implements the two most cited regression models in obstetrics:

Shepard (1982): Uses Biparietal Diameter (BPD) and Abdominal Circumference (AC).
Hadlock (1985): Uses BPD and AC (and optionally FL/HC, though this tool focuses on the BPD/AC core model).

These formulas are mathematically derived from thousands of fetal observations and are the standard of care in modern radiology.

Formulas

The Shepard formula is logarithmic, while Hadlock uses a polynomial regression. Both result in an estimated weight in grams.

Log₁₀W = −1.7492 + 0.166(BPD) + 0.046(AC) − 2.646 AC × BPD1000

Reference Data

Measurement	Abbreviation	Description
Biparietal Diameter	BPD	The diameter of the fetal head measured from one parietal bone to the other.
Abdominal Circumference	AC	The circumference of the fetal abdomen at the level of the liver and stomach. Most sensitive indicator of fetal nutrition.
Femur Length	FL	The length of the thigh bone. (Used in extended Hadlock formulas).
Head Circumference	HC	The circumference of the skull.

Frequently Asked Questions

Shepard is often preferred for simpler biometry when only head and abdomen measurements are available. It is widely validated but can be slightly less accurate than the 4-parameter Hadlock model at extremes of weight.

Ultrasound weight estimation generally has a margin of error of ±15% to ±20%. This means a baby estimated at 3000g could realistically be between 2550g and 3450g.

Yes, these formulas apply to each fetus individually. However, measuring AC accurately in crowded multiple gestations can be technically challenging, potentially increasing error.