Gestation is the carrying of an embryo or fetus insie a female viviparous animal. Mammals during pregnancy can have one or more gestations at the same time (multiple gestations). The time interval of a gestation plus 2 weeks is called gestation period, and the length of time plus 2 weeks that the offspring have spent developing in the uterus is called gestational age. (The extra 2 weeks is because gestational age is counted starting from the last menstrual period (LMP), rather than actual conception. However this extra 2 weeks is not always added when talking about animals.)