The uterus is shaped like an upside-down pear, about 3 inches long by 2 inches wide by 1 inch deep (7.5 cm by 5 cm by 2.5 cm), superior to the urinary bladder and between the two ovaries in the pelvic cavity (see Fig. 20–5). The broad ligament also covers the uterus (see Fig. 20–6). During pregnancy the uterus increases greatly in size, contains the placenta to nour-ish the embryo-fetus, and expels the baby at the end of gestation.
The parts and layers of the uterus are shown in Fig. 20–6. The fundus is the upper portion above the entry of the fallopian tubes, and the body is the large central portion. The narrow, lower end of the uterus is the cervix, which opens into the vagina.
The outermost layer of the uterus, the serosa or epimetrium, is a fold of the peritoneum. The myome-trium is the smooth muscle layer; during pregnancy these cells increase in size to accommodate the grow-ing fetus and contract for labor and delivery at the end of pregnancy.
The lining of the uterus is the endometrium, which itself consists of two layers. The basilar layer, adjacent to the myometrium, is vascular but very thin and is a permanent layer. The functional layer is regenerated and lost during each menstrual cycle. Under the influence of estrogen and progesterone from the ovaries, the growth of blood vessels thick-ens the functional layer in preparation for a possible embryo. If fertilization does not occur, the functional layer sloughs off in menstruation. During pregnancy, the endometrium forms the maternal portion of the placenta.