uterus /yo̅o̅″tərəs/ [L, womb] , the hollow pear-shaped internal female organ of reproduction in which the fertilized ovum is implanted and the fetus develops and from which the decidua of menses flows. Its anterior surface lies on the superior surface of the bladder, separated by a fold of peritoneum, the vesicouterine pouch. Its posterior surface, also covered with peritoneum, is adjacent to the sigmoid colon and some of the coils of the small intestine. The uterus is composed of three layers: the endometrium, the myometrium, and the parametrium. The endometrium lines the uterus and becomes thicker and more vascular in pregnancy and during the second half of the menstrual cycle under the influence of the hormone progesterone. The myometrium is the muscular layer of the organ. Its muscle fibers wrap around the uterus obliquely, laterally, and longitudinally. The muscle fibers contract during childbirth to expel the fetus. After childbirth the meshlike network of fibers contracts again, creating a mass of natural ligatures that stops the flow of blood from the large blood vessels supplying the placenta. The parametrium is the outermost layer of the uterus. It is composed of serous connective tissue and extends laterally into the broad ligament. In the adult the organ measures about 7.5 cm long and 5 cm wide at its fundus and weighs approximately 40 g. During pregnancy it is able to grow to many times its usual size, almost entirely by cellular hypertrophy.
