Biological motherhood for humans, as in other mammals, occurs when a pregnant female gestates a fertilized ovum (the "egg"). Typically a fetus develops from the viable zygote, resulting in an embryo. Gestation occurs in the woman's uterus until the fetus (assuming it is carried to term) is sufficiently developed to be born. In humans, gestation is often around 9 months in duration, after which the woman experiences labor and gives birth. This is not always the case, however, as some babies are born prematurely, late, or in the case of stillbirth, do not survive gestation. Usually, once the baby is born, the mother produces milk via the lactation process. The mother's breast milk is the source of antibodies for the infant's immune system and commonly the sole source of nutrition for the first year or more of the child's life.[3][4][5]