Pregnancy is the condition of a female animal while young are developing within her uterus.

The interval, the gestation period, extends from fertilization of the ovum to the birth of the offspring. It includes fertilization, the union of the ovum and sperm; early embry­onic development in the lumen of the female reproductive tract; implantation of the embryo in the uterine wall; placentation, the develop­ment of fetal membranes; and continued growth of the fetus.

Normal gestation periods vary greatly from species to species (see Table 27-1), and there is considerable variation between individuals within each species. If the young are carried throughout a normal gestation period, it is a full-term pregnancy. A premature birth is deliv­ery of a viable fetus before fetal development is complete. Termination of pregnancy with deliv­ery of a nonviable fetus is abortion.