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In pregnancy, the uterine cervix serves 2 major functions. First, it maintains its firmness (ie, physical integrity) during pregnancy as the uterus dramatically enlarges. This physical integrity is critical so that the developing fetus can remain in the uterus until the appropriate time for delivery. Second, in preparation for labor and delivery, the cervix softens and becomes more distensible, a process called cervical ripening. These chemical and physical changes are required for cervical dilatation, labor, and delivery of a fetus.

The human uterus is composed of 2 basic parts, the fundus and the cervix. The fundus is composed of myometrium, predominantly smooth muscle cells, and the endometrium. The endometrium undergoes dramatic changes during the menstrual cycle. In the absence of pregnancy, it sheds down to the basal layer at the end of the cycle. The normal pregnant cervix is 3.5 cm or longer and is composed predominantly of connective tissue, mainly collagen. In contrast to the fundus, it has only 10-15% smooth muscle. It changes little during the menstrual cycle and pregnancy until the onset of cervical ripening.

The human cervix consists mainly of extracellular connective tissue. The predominant molecules of this extracellular matrix are type 1 and type 3 collagen, with a small amount of type 4 collagen at the basement membrane. Intercalated among the collagen molecules are glycosaminoglycans and proteoglycans, predominantly dermatan sulfate, hyaluronic acid, and heparin sulfate. Fibronectin and elastin also run among the collagen fibers. The highest ratio of elastin to collagen is at the internal os. Both elastin and smooth muscle decrease from the internal to the external os of the cervix.

Cervical ripening usually begins prior to the onset of labor contractions and is necessary for cervical dilatation and the passage of the fetus. Cervical ripening is the result of a series of complex biochemical processes that ends with rearrangement and realignment of the collagen molecules. The cervix thins, softens, relaxes, and opens in response to uterine contractions, which pull the cervix over the presenting fetal part.

In late pregnancy, hyaluronic acid content increases in the cervix. This leads to an increase in water molecules that intercalate among the collagen fibers. The amount of dermatan sulfate decreases, which leads to reduced bridging among the collagen fibers and, thus, a decrease in cervical firmness. Chondroitin sulfate also decreases.

Cervical ripening is associated with decreased collagen fiber alignment, decreased collagen fiber strength, and diminished tensile strength of the extracellular cervical matrix. An associated change with the cervical ripening process is an increase in cervical decorin (dermatan sulfate proteoglycan 2), which leads to collagen fiber separation. All of these changes cause cervical softening (ie, ripening). With uterine contractions, the ripened cervix dilatates, leading to reorientation of the tissue fibers in the cervix in the direction of the stress. Under the effect of myometrial contractions, the cervix passively dilatates and is pulled over the presenting fetal part. Evidence also indicates that the elastin component of the cervix behaves in a ratchetlike manner so that dilatation is maintained following the contraction.

In summary, cervical ripening is the result of realignment of collagen, degradation of collagen cross-linking due to proteolytic enzymes, and dilatation resulting from these processes plus uterine contractions. This is a complicated series of events in which many things occur both simultaneously and sequentially. Research in this area is complex because studying human subjects is difficult and many differences exist between species.

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