The ultrasound technologist measures the head width, head circumference, abdominal circumference, femur length, and occasionally other measurements. These measurements are then entered into a formula
which calculates the estimated fetal weight. This formula and estimation are based on population averages and is therefore subject to normal variation. The estimated fetal weight is quite accurate early on during pregnancy, and less accurate as the pregnancy progresses. At full term, the actual fetal weight can vary by more than 1 pound. The estimated fetal weight is useful to the obstetrician in order to indicate whether or not baby is growing appropriately.