In the nonpregnant state, the cervix comprises the lower one-third of the uterus and connects the uterine cavity to the vagina. The overall length of the cervix is highly variable but generally falls in the range of 2.5 to 5.0 cm. Only about one-half to one-third of the cervix extends into the vagina and this is called the portio vaginalis. The portion of the cervix that opens into the vagina is called the external cervical os; the uppermost portion that opens into the uterus is called the internal cervical os. As we will emphasize in later discussion, the internal cervical os is a key player in the etiology, pathogenesis, and diagnosis of cervical incompetence. The portion of the cervix that connects the external os and internal os contains the endocervical canal and this narrow tube separates the relatively unsterile environment of the vagina from the uterine cavity. The endocervical canal is lined by a single layer of mucous producing cells and the chemicals and immunoglobulins that are secreted into the canal provide the major barrier to ascending infection by potential pathogenic microorganisms from the vagina and ectocervix.
Although the cervix is contiguous with the body of the uterus, it is structurally different from the uterus in several key aspects. Both the uterine wall (the myometrium) and the cervix contain smooth muscle and fibrous connective tissue, but there is a much greater percentage of the connective tissue in the cervix than in the myometrium. The uterus is “designed” to contract and, when the time is right, eventually push the baby out, while the role of the cervix, under normal circumstances, is to keep the baby inside until it is mature enough to survive in the cold cruel world outside the womb.
The fibrous connective tissue of the cervix is mostly composed of types I and II collagen, elastin, and proteoglycans. The collagen is heavily ‘cross-linked’ and this imbues the cervix with a tremendous resistance, again under normal circumstances, to stretching and ‘softening’ until the biochemical cascade that progresses to labor ensues. At that point, the cervix is capable of undergoing a remarkably rapid transformation from a structure that has the consistency of a rubber eraser to the soft, compliant, elastic structure that will permit the relatively easy passage of the baby from the uterus and into the birth canal – a transformation that results from the remodeling (uncross-linking) of the collagen and the extracellular matrix.
The cervix, then, is a very dynamic organ that always has the innate ‘potential’ to change from barrier to facilitator of uterine evacuation. In cervical incompetence, some women probably do not have a normal percentage or configuration of the fibrous connective tissue, others may be overly sensitive, due to genetic susceptibility or metabolic abnormalities, to stimuli that would ordinarily not lead the “normal" cervix to undergo the transformations associated with premature cervical ripening, others may be exposed to higher levels of factors that can initiate cervical ripening independently of the innate pathways, and others may have congenital or iatrogenic diminution of normal ‘integrity’ at the internal os that leads to subclinical cervical changes that eventually result in activation of the innate ‘ripening’ pathways or to ascending infection that can promote more rapid cervical change and/or premature uterine contractions. Then of course, there may be individuals who have more than one of these predisposing factors...