gestation /jestā″shən/ [L, gestare, to bear] , in a viviparous animal, the period from the fertilization of the ovum until birth. Gestation varies with the species. In humans the average duration is 266 days, or approximately 280 days from the onset of the last menstrual period. A gestation time of less than 37 weeks is regarded as premature; one that continues beyond 42 weeks is considered postmature, regardless of the size of the fetus or other factors. See also pregnancy.
