Gestational age is the number of weeks and days a fetus has developed since the beginning of the pregnancy, or gestation. A pregnancy is formally considered as beginning on the first day of the mother's last menstrual period (LMP).
An infant born before 37 completed weeks' gestation is considered premature. A premature infant is often referred to in terms of gestational age (a "30-weeker"), which implies a particular point of development at birth. After a premature infant is born, the gestational age is also referred to as the infant's postconceptional age. This figure is useful for estimating an infant's growth and development until some time around the due date (40 weeks).
If the mother's LMP is not known, gestational age is estimated by using ultrasound measurements of the fetus combined with the dates of first fetal heart tones and other developmental milestones. After the infant is born, there are a variety of characteristics that can be used to estimate the gestational age.
It is possible for gestational age to be inaccurate by up to 2 weeks, even with an accurate LMP date confirmed by other tests.
WebMD Medical Reference from Healthwise