From Encyclopedia of Sex and Sexuality
The female uterus, or womb, is the organ within which a newly fertilized egg is implanted to begin its development through successive stages as an embryo, a fetus, and at birth, a newborn infant.
The uterus, an organ with roughly the size and shape of a pear—except during pregnancy—is situated in the woman’s lower abdomen at the internal and upper end of the vagina. The lower opening of the uterus, the cervix, can be exposed at the time of a physician’s pelvic examination with the aid of an instrument called a speculum, which gently separates the walls of the vagina. The opening of the cervix, called the cervical canal, leads to the interior lining of the uterus, the endometrium. Within the uterine cavity are two more openings, the canals of the fallopian tubes, through which the egg or ovum passes after fertilization to reach the uterus.
Pregnancy is considered to begin with implantation of a newly fertilized egg in the endometrium. The uterus will provide the environment and protection necessary as a new pregnancy begins the complex process of growth through the nine months of gestation required by a full-term infant. The blood vessels to the uterus develop to make it possible for the mother’s blood supply to provide oxygen and other metabolic and nutritional substances needed for growth and development as the fertilized egg grows into the embryonic and then fetal stages. These life-supporting substances are transmitted to the fetus through the placenta, a fluid-filled sac that forms within the uterus and around the developing embryo and cushions and protects it. The uterus and placenta expand to many times their initial size during the course of pregnancy.
In what may seem a contradiction to its ability to stretch to many times its usual size during pregnancy, the uterus is primarily a muscular organ. The muscular component surrounding the endometrium is called the myometrium. During labor, the myometrial contractions force the emerging baby down through the dilated cervix and vagina toward delivery. In the nonpregnant woman, myometrial contractions are the cause of menstrual cramps (dysmenorrhea). Prostaglandins, natural hormones, initiate myometrial contractions, and pitocin, a pituitary hormone, also causes uterine contractions.
When a sexually mature woman is not pregnant, her uterus responds to the cyclic hormonal secretions of her ovaries, almond-sized organs on either side of the uterus that contain her ova. Once each month an ovary releases one ovum (egg) into a fallopian tube for potential fertilization. Prior to ovulation, the point in the menstrual cycle when the ovum is released by the ovary, the lining of the uterus is built up in response to increasing levels of the hormone estrogen. After ovulation, the endometrium is kept in a receptive state for the possible implantation of a fertilized ovum by increased levels of progesterone and estrogen. If an ovum does not implant, there is an abrupt drop in these hormone levels, the small arteries supplying the uterine lining (the endometrium) constrict, and the lining disintegrates and is shed with the bleeding we call “menstruation.” Women usually begin having menstrual periods sometime after the age of ten and continue to have them at regular intervals until middle age. The time when menstrual periods cease is called menopause.