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%).