Anatomy of the cervix. The cervix is the lower, narrow end of the uterus that connects the uterus to the vagina. It is made up of the internal OS (the opening between the cervix and the upper part of the uterus), the endocervix (the inner part of the cervix that forms the endocervical canal), the ectocervix (the outer part of the cervix that opens into the vagina) and the external OS (the opening between the cervix and vagina). The area where the endocervix and ectocervix meet is called the squamocolumnar junction, which contains both glandular cells (column-shaped cells that make mucus) from the endocervix and squamous cells (thin, flat cells) from the ectocervix. The squamocolumnar junction is sometimes referred to as the transformation zone.

Cervix is the elongate inferior end of the uterus where it connects to the vagina.

The Uterus and Cervix

The uterus is the muscular organ that nourishes and supports the growing embryo (see Figure). Its average size is approximately 5 cm wide by 7 cm long (approximately 2 in by 3 in) when a female is not pregnant. It has three sections. The portion of the uterus superior to the opening of the uterine tubes is called the fundus. The middle section of the uterus is called the body of uterus (or corpus). The cervix is the narrow inferior portion of the uterus that projects into the vagina. The cervix produces mucus secretions that become thin and stringy under the influence of high systemic plasma estrogen concentrations, and these secretions can facilitate sperm movement through the reproductive tract.

Several ligaments maintain the position of the uterus within the abdominopelvic cavity. The broad ligament is a fold of peritoneum that serves as a primary support for the uterus, extending laterally from both sides of the uterus and attaching it to the pelvic wall. The round ligament attaches to the uterus near the uterine tubes, and extends to the labia majora. Finally, the uterosacral ligament stabilizes the uterus posteriorly by its connection from the cervix to the pelvic wall.

This anterior view shows the relationship of the ovaries, uterine tubes (oviducts), and uterus. Sperm enter through the vagina, and fertilization of an ovulated oocyte usually occurs in the distal uterine tube. From left to right, LM × 400, LM × 20. (Micrographs provided by the Regents of University of Michigan Medical School © 2012)

National Cancer Institute (NCI)

If the cervix is not ready for labor, a health care provider may suggest one of the following steps to ripen the cervix:

• Stripping the membranes. The health care provider can separate the thin tissue of the amniotic sac, which contains the fetus, from the wall of the uterus. This process causes the body to release prostaglandins (pronounced pros-tuh-GLAN-dins), which soften the cervix and cause contractions.
• Giving prostaglandins. This drug may be inserted into the vagina or given by mouth. The body naturally makes these chemicals to ripen the cervix, but sometimes additional amounts are needed to help labor occur.
• Inserting a catheter. A small tube with an inflatable balloon on the end can be placed in the cervix to widen it.

A change in the CERVIX UTERI with respect to its readiness to relax. The cervix normally becomes softer, more flexible, more distensible, and shorter in the final weeks of PREGNANCY. These cervical changes can also be chemically induced (LABOR, INDUCED).

Stages of Childbirth

The process of childbirth can be divided into three stages: cervical dilation, expulsion of the newborn, and afterbirth (Figure 28.21).

Cervical Dilation

For vaginal birth to occur, the cervix must dilate fully to 10 cm in diameter—wide enough to deliver the newborn’s head. The dilation stage is the longest stage of labor and typically takes 6–12 hours. However, it varies widely and may take minutes, hours, or days, depending in part on whether the mother has given birth before; in each subsequent labor, this stage tends to be shorter.

Figure 28.21 Stages of Childbirth The stages of childbirth include Stage 1, early cervical dilation; Stage 2, full dilation and expulsion of the newborn; and Stage 3, delivery of the placenta and associated fetal membranes. (The position of the newborn’s shoulder is described relative to the mother.)

True labor progresses in a positive feedback loop in which uterine contractions stretch the cervix, causing it to dilate and efface, or become thinner. Cervical stretching induces reflexive uterine contractions that dilate and efface the cervix further. In addition, cervical dilation boosts oxytocin secretion from the pituitary, which in turn triggers more powerful uterine contractions. When labor begins, uterine contractions may occur only every 3–30 minutes and last only 20–40 seconds; however, by the end of this stage, contractions may occur as frequently as every 1.5–2 minutes and last for a full minute.

Each contraction sharply reduces oxygenated blood flow to the fetus. For this reason, it is critical that a period of relaxation occur after each contraction. Fetal distress, measured as a sustained decrease or increase in the fetal heart rate, can result from severe contractions that are too powerful or lengthy for oxygenated blood to be restored to the fetus. Such a situation can be cause for an emergency birth with vacuum, forceps, or surgically by Caesarian section.

The amniotic membranes rupture before the onset of labor in about 12 percent of women; they typically rupture at the end of the dilation stage in response to excessive pressure from the fetal head entering the birth canal.