Fetal Weight Estimator

Estimated Fetal Weight (EFW) is calculated by ultrasound using measurements of the baby’s head, abdomen, and femur. The most widely used clinical formula is the Hadlock formula, developed from large population studies and implemented in virtually all modern obstetric ultrasound systems.

Hadlock Formula (1985 — most common clinically): log₁₀(EFW) = 1.3596 − 0.00386(AC×FL) + 0.0064(HC) + 0.00061(BPD×AC) + 0.0424(AC) + 0.174(FL)

For clinical purposes, simplified versions using 2–3 measurements are also used:

Two-parameter Hadlock (AC + FL): log₁₀(EFW) = 1.304 + 0.05281(AC) + 0.1938(FL) − 0.004(AC×FL)

What each variable means:

• AC (Abdominal Circumference): measured at the level of the fetal liver and stomach. The single best predictor of fetal size and nutritional status. Units: millimeters.
• FL (Femur Length): the length of the thigh bone. Correlates with overall skeletal growth. Units: millimeters.
• HC (Head Circumference): circumference of the fetal skull. Reflects brain growth. Units: millimeters.
• BPD (Biparietal Diameter): the width of the skull from side to side. Combined with HC for head growth. Units: millimeters.
• EFW: result in grams.

Average fetal weight by gestational age:

Worked example (simplified two-parameter): Ultrasound at 32 weeks: AC = 283 mm, FL = 62 mm.

log₁₀(EFW) = 1.304 + 0.05281(283) + 0.1938(62) − 0.004(283 × 62) = 1.304 + 14.945 + 12.016 − 70.184 (converting: 0.004 × 17,546 = 70.184) Note: All measurements in cm for this formula: AC = 28.3 cm, FL = 6.2 cm log₁₀(EFW) = 1.304 + 0.05281×28.3 + 0.1938×6.2 − 0.004×28.3×6.2 = 1.304 + 1.4945 + 1.2016 − 0.7018 = 3.2983 EFW = 10^3.2983 = ~1,987g ≈ 1.99 kg (within normal 32-week range of 1,400–2,100g)

Important: EFW has a margin of error of ±10–20%. It is a screening estimate, not a precise measurement.