Gestation is the carrying of an embryo or fetus inside female viviparous animals. It is typical for mammals, but also occurs for some non-mammals. Mammals during pregnancy can have one or more gestations at the same time (multiple gestations).The time interval of a gestation is called the gestation period. In human obstetrics, gestational age refers to the embryonic or fetal age plus two weeks. This is approximately the duration since the woman's last menstrual period (LMP) began.