Pregnancy (Cross-section View)

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During pregnancy, the uterus or womb is a hollow, muscular organ in which a fertilized egg, called the zygote, becomes embedded. 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 20% of women, it tilts backwards.

The uterus is lined with tissues that 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. The endometrium is the membrane that lines the uterus. It is a glandular mucous membrane, and is formed during the menstrual cycle, then expelled from the body during each menstruation. This occurs in females on average every twenty-eight days, but varies with each person. As the endometrium is expelled, the blood vessels which connect the endometrial lining to the uterus break, causing bleeding and usually a small degree of pain. The endometrium is what a fertilized egg attaches to upon fertilization, so it is not shed if a fertilized egg is implanted. The endometrium also supplies blood for the fetus and allows it to receive oxygen.

During pregnancy, the uterus stretches from three to four inches in length to a size that 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.