The period that the fetus develops from conception to birth.

The human gestation period—the period of time between conception and labor—is approximately forty weeks (280 days), measured from the first day of the mother's last menstrual period. A gestation period of thirty-eight weeks (266 days) is calculated for women who are pregnant by a procedure such as in vitro fertilization or artificial insemination that allows them to know their exact date of conception. The gestational period is divided into three major periods called trimesters. The first trimester last from weeks one through twelve, the second from weeks thirteen through twenty-seven, and the third from weeks twenty-eight to forty. The gestational age of a fetus or newborn infant corresponds to how far along it is in the gestation period, usually measured in weeks and days from the first day of the...