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Labor is characterized by forceful and painful uterine contractions that effect cervical dilation and cause the fetus to descend through the birth canal. However, extensive preparations take place in both the uterus and cervix long before this. During the first 36 to 38 weeks of normal gestation, the myometrium is in a preparatory yet unresponsive state. Concurrently, the cervix begins an early stage of remodeling yet maintains structural integrity. Following uterine quiescence, a transitional phase begins during which myometrial unresponsiveness is suspended and the cervix undergoes ripening, effacement, and loss of structural cohesion.

The physiological processes that regulate parturition and the onset of labor continue to be defined. Three theories describe labor initiation. Viewed simplistically, the first is the functional loss of pregnancy maintenance factors. The second focuses on synthesis of factors that induce parturition. The third suggests that the mature fetus is the source of the initial signal for parturition commencement. Research supports a model that draws from all three themes. Thus, labor onset represents the culmination of a series of biochemical changes in the uterus and cervix. These result from endocrine and paracrine signals emanating from both mother and fetus. Their relative contributions vary between species, and it is these differences that complicate elucidation of the exact factors that regulate human parturition. When parturition is abnormal, preterm labor, dystocia, or postterm pregnancy may result.

MATERNAL AND FETAL COMPARTMENTS

Uterus and Cervix

The myometrial layer of the uterus is composed of bundles of smooth muscle cells surrounded by connective tissue. In contrast to skeletal or cardiac muscle, the smooth muscle cell is not terminally differentiated and therefore is readily adaptable to environmental changes. Varied stimuli such as mechanical stretch, inflammation, and endocrine and paracrine signals modulate the transition of the smooth muscle cell into phenotypes that provide cell growth, proliferation, secretion, and contractility.

Additionally, several smooth muscle qualities confer advantages for uterine contraction efficiency and fetal delivery. First, the degree of smooth muscle cell shortening with contractions may be one order of magnitude greater than that attained in striated muscle cells. Second, forces can be exerted in smooth muscle cells in multiple directions. This differs from the contraction force generated by skeletal muscle, which is always aligned with the axis of the muscle fibers. Third, the thick and thin filaments of smooth muscle are found in long, random bundles throughout the cells. This plexiform arrangement aids augmented shortening and force-generating capacity. Last, greater multidirectional force generation in the uterine fundus compared with that of the lower uterine segment helps optimize expulsive force vectors.

Lining the thick muscular uterine walls, the endometrium is transformed by pregnancy hormones and is then termed decidua. Composed of stromal cells and maternal immune cells, the decidua serves to maintain the pregnancy via unique immunoregulatory functions that suppress inflammatory signals during gestation. However, at the end of ...

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