The uterus is part of the reproductive system of women and all female mammals. It is a hollow organ in the pelvis in which a baby grows, and many sources describe it as pear-shaped. In women - and I guess in all mammals - the lower, narrow opening into the uterus from the vagina is called the cervix. The broad middle part is the body or corpus, while the top is called the fundus. Extending on either side of the uterus are the fallopian tubes, which connect to the ovaries. The wall of the uterus has two layers of tissue: an inner mucus membrane lining called the endometrium, and an outer layer of muscle tissue called the myometrium.
Each month the endometrium of a woman of childbearing age becomes engorged with blood to prepare for the implantation of a fertilized egg. If a woman becomes pregnant, the fetus will lodge itself in this layer and begin to grow. If she does not, her production of progesterone, the hormone which maintains the engorged layer, drops, and the lining is sloughed off. The myometrium contracts to push the thick bloody lining out of the body through the vagina. It is these contractions which cause menstrual cramps, and this bloody expulsion which is known as menstruation, a period, the curse, the little red sister, and countless other affectionate or resentful names.