The uterus or womb is the major female reproductive organ of most mammals, including humans. One end called the cervix, opens into the vagina, and the wider end, called the body of uterus, is connected on both sides with the Fallopian tubes. It comes in a variety of shapes and sizes in different organisms. In humans it is pear-shaped. Some organisms such as rabbits, pigs, goats, and horses have bipartite or "horned" uteri.
The uterus mostly consists of muscle. The innermost permanent layer of tissue is called the endometrium. In most mammals, including humans, the endometrium builds a lining periodically which, if no pregnancy occurs, is shed or reabsorbed. Shedding of the endometrial lining in humans is responsible for menstrual bleeding (known colloquially as a woman's "period") throughout the fertile years of a female. In other mammals there may be cycles set as widely apart as six months or as frequently as a few days.
The main function of the uterus is to accept a fertilized ovum which becomes implanted into the endometrium, and derives nourishment from blood vessels which develop exclusively for this purpose. The fertilized ovum becomes an embryo, develops into a fetus and gestates until childbirth. Due to anatomical barriers such as the pelvis, the uterus is pushed partially into the abdomen due to its expansion during pregnancy. Even in pregnancy the mass of a human uterus amounts to only about a kilogram (2.2 pounds).
