This is a response to VC 20 Stuart 1 ("Yea!")...
Birthweight is predicted from measurements taken during the ultra-sound. It is based primarily upon the head and abdomen circumference and femur length. These measures are then compared to average sizes for fetuses of that age.
However, inaccuracies can abound in these predictions. First of all, age of the fetus can be off by as much as two weeks. Second, the measurements taken during ultrasounds can be inaccurate. It depends on the skill of the technician (measurements are drawn like you would draw a line in MacDraw) and the position of what is being measured because all ultrasounds are two dimensional. For example, the femur would have to be perfectly perpendicular to the ultrasound waves that are bouncing off of it in order for completely accurate measurements to be obtained.