Gestation is the period of development of an embryo or fetus inside a viviparous animal. Learn about the gestation period in humans, mammals, non-mammals and other animals, and how it affects health and lifespan. gestation , in mammals, the time between conception and birth, during which the embryo or fetus is developing in the uterus. This definition raises occasional difficulties because in some species (e.g., monkeys and man) the exact time of conception may not be known. In these cases the beginning of gestation is usually dated from some well-defined point in the reproductive cycle (e.g., the beginning of the previous menstrual period ). Explore the human gestation period and fetal development timeline, including the duration of pregnancy and the stages of human embryonic development. Gestation is the period of time between conception and birth when a baby grows and develops inside the mother's womb. Because it's impossible to know exactly when conception occurs, gestational age is measured from the first day of the mother's last menstrual cycle to the current date.
