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, esp. if the date of the last menstrual period is either vaguely remembered or unknown.
Use of ultrasound to measure the crown-rump length in the first trimester, the biparietal diameter in the second trimester, and other measurements permits a more nearly precise estimate of gestational age. Even so, these techniques, because of biological variation of fetal size and early intrauterine growth failure, may not be consistently accurate.