Gestational age is usually considered to be the age of an embryo or fetus (or newborn infant) from the first day of the woman's last menstrual period (LMP). This standard system of counting the progression of pregnancy starts approximately two weeks before fertilization takes place; it does not in itself constitute the beginning of pregnancy. There is some controversy over the beginning of pregnancy, and alternative counting systems have been suggested.
A full-term human pregnancy is considered to be 40 weeks (280 days), though pregnancy lengths between 38 and 42 weeks are considered normal. A fetus born prior to the 37th week of gestation is considered premature and faces increased risk of morbidity and mortality. An approximation of the due date is given by Naegele's rule.
The gestational age should not be confused with the fertilization age, (also called conceptional age or developmental age) of an embryo or fetus; the fertilization age is always counted from fertilization, and the gestational age is usually greater by about two weeks.
The events of prenatal development usually occur at known gestational ages. The gestational timing of a toxin exposure or infection can be used to predict the potential consequences to the fetus.
Calculations of gestational age from LMP are sometimes incorrect due to normal variation from the average ovulation date. The gestational age of an individual infant can be more accurately estimated from:
The fertilization age of children conceived by in vitro fertilization is known to the hour.