The uterine cavity is also called the uterus, or womb; it is a hollow, muscular organ in which a fertilized egg, called the zygote, becomes embedded and in which the egg is nourished and allowed to develop until birth. It lies in the pelvic cavity behind the bladder and in front of the bowel. The uterus usually tilts forward at a ninety degree angle to the vagina, although in about twenty percent ofmycontentbreak women it tilts backwards. The uterus is lined with tissues which change during the menstrual cycle. These tissues build under the influence of hormones from the ovary. When the hormones withdraw after the menstrual cycle, the blood supply is cut off and the tissues and unfertilized egg are shed as waste. During pregnancy, the uterus stretches from three to four inches in length to a size which will accommodate a growing baby. During this time, muscular walls increase from two to three ounces to about two pounds and these powerful muscles release the baby through the birth canal with great force. The womb shrinks back to half its pregnant weight before a baby is a week old. By the time the baby is a month old, the uterus may be as small as when the egg first entered.