Chapter 4: Uterus and Vagina

UTERUS

The normal uterus is a small muscular organ in the female pelvis. It is composed of three layers:

1. An outer, covering, serous peritoneal layer—the perimetrium

2. A thick middle layer made up of muscle fibers—the myometrium

3. An inner mucous layer of glands and supporting stroma—the endometrium—which is attached directly to the myometrium

The myometrium is made up of three layers of muscle:

1. An outer layer of mainly longitudinal fibers

2. An inner layer whose fibers run, for the most part, in a circular direction

3. A thick middle layer whose fibers are arranged in an interlacing pattern and through which the blood vessels course. When these fibers contract and retract after the products of conception have been expelled, the blood vessels are kinked and constricted. In this way, postpartum bleeding is controlled

Uterine Shape

In the nonpregnant condition and at the time of implantation, the uterus is pear shaped. By the third month of gestation, the uterus is globular. From the seventh month to term, the contour is again pyriform.

Uterine Size

The uterus grows from the nonpregnant dimensions of about 7.5 × 5.0 × 2.5 cm to 28 × 24 × 21 cm. The weight rises from 30 to 60 g to 1000 g at the end of pregnancy. The uterus changes from a solid organ in the nullipara to a large sac, the capacity increasing from almost nil to 4000 mL.

Uterine Location

Normally, the uterus is entirely in the pelvis. As it enlarges, it gradually rises, and by the fourth month of gestation, it extends into the abdominal region.

Uterine Divisions

1. The fundus (Fig. 4-1) is the part superior to the openings of the fallopian tubes

2. The body (corpus) is the main part; it has thick walls, lies between the tubal openings and the isthmus, and is the main contractile portion. During labor, the contractions force the baby downward, distend the lower segment of the uterus, and dilate the cervix

3. The isthmus is a small constricted region of the uterus. It is about 5 to 7 mm in length and lies superior to the internal os of the cervix

4. The cervix (Fig. 4-2) is composed of a canal with an internal os superiorly, separating the cervix from the uterine cavity, and an external os inferiorly which closes off the cervix from the vagina. The cervix is about 2.5 cm in length. The lower part pierces the anterior wall of the vagina, and its tissue blends with that of the vagina