This estimated fetal weight calculator will calculate percentiles as well as the estimated fetal weights based ultrasound data and on many published formulas. Formulas include those by Hadlock, Shepard, Woo, Shinozuka, Ott, Combs, Warsof, Campbell, and many others.
Calculations are based on the 4 common fetal measurements, biparietal diameter (BPD), head circumference (HC), femur length (FL), and abdominal circumference (AC). Many studies have devised different formulas. In general, about 2 in 3 actual fetal weights will be about 16% plus/minus the average. Actual weights can often be higher or lower than the 16% +/-.
One standard deviation ( 1 SD) encompasses about 68% of all cases, so you should expect about 2 in 3 babies to weight within the range of one standard deviation range. Two standard deviations include about 95% of all patients, so the range of two standard deviations (or 2 SD) would include 95% of all actual weights, but still about 5% of babies would either be bigger (2.5%) or smaller (2.5%).