Determination of the prenatal age of the fetus. This information is essential for obstetrical care because it influences the decision to intervene and at what time. The age has been estimated by evaluating the menstrual history, time of initial detection of fetal heart tones, and date the level of the fundus reaches the umbilicus. These methods are not precise, especially if the date of the last menstrual period is either vaguely remembered or unknown.