Fetal Weight Percentile Calculator

This calculator relies on established obstetric algorithms that correlate ultrasound measurements with fetal mass. The primary computation uses a logarithmic regression formula to estimate the weight based on standard biometric parameters: log(weight) = 1.3596 - (0.00386 * AC * FL) + (0.0064 * HC) + (0.00061 * BPD * AC) + (0.0424 * AC) + (0.174 * FL). Here, AC represents Abdominal Circumference, FL is Femur Length, HC stands for Head Circumference, and BPD is Biparietal Diameter. Once the estimated fetal weight (EFW) in grams is derived, it is compared against gestational age and sex-specific normative growth databases to determine the precise percentile ranking.

Imagine a routine third-trimester ultrasound at 32 weeks of gestation reveals the following measurements for a male fetus: an Abdominal Circumference (AC) of 28.0 cm, a Femur Length (FL) of 6.2 cm, a Biparietal Diameter (BPD) of 8.1 cm, and a Head Circumference (HC) of 29.5 cm. First, we substitute these values into the logarithmic weight formula: log(weight) = 1.3596 - (0.00386 * 28.0 * 6.2) + (0.0064 * 29.5) + (0.00061 * 8.1 * 28.0) + (0.0424 * 28.0) + (0.174 * 6.2). Solving this equation yields a log weight of approximately 3.268, which translates to an Estimated Fetal Weight (EFW) of roughly 1,854 grams. Comparing this 1,854 gram weight against standard 32-week male growth charts places the baby precisely at the 50th percentile, indicating healthy, median growth.

To ensure the most reliable percentile results, always use recent measurements taken by a certified ultrasound technician or healthcare provider. Ultrasound biometry can have a natural margin of error of 10 to 15 percent in weight estimation, so single measurements should be viewed as part of an overall growth trajectory rather than an absolute value. Regular follow-up scans spaced a few weeks apart offer a much clearer picture of your baby's sustained health and development than any single isolated reading.