A method for determining the volume of an irregularly shaped body part or body such as a fetus using a 3D ultrasonic image composed of light and dark pixels. A caliper is placed on a portion whose volume is to be determined and a computer having the image, automatically counts every pixel with the same echogenicity in every direction as long as it is continuous with the spot the caliper is in, and wherein the computer means stops counting in any direction where the pixels become significantly more echogenic (dark). The volume of the portion, is determined, based on the counted number of pixels and the magnification of the image; and the calculated volume is used to determine physiological characteristics based on known values such as fetal gestation age.