The estimated fetal weight is the baby's weight at a specific time of the pregnancy. Because the measurement cannot be taken directly, it is estimated using an ultrasound scan, which finds:
Abdominal and head circumference (AC and HC);
Biparietal diameter (the distance between parietal bones in the skull) (BPD); and
Femur length (occasionally also humerus length) (HL).
Plug those measurements in the following formula to find the estimated fetal weight:
log₁₀(Fetal weight) = 1.3596 - (0.00386 × AC × FL) + (0.0064 × HC) + (0.00061 × BPD × AC) + (0.0424 × AC) + (0.174 × FL). Read more