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From time immemorial inquiring minds have sought an explanation for the fact that labour usually ensues about 280 days after the appearance of the last menstrual period, but thus far no satisfactory universal cause has been discovered.

—J. Whitridge Williams (1903)

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INTRODUCTION

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The importance of labor physiology was highlighted in the first edition of Williams Obstetrics, in which an entire section was devoted to the topic. Given the science at that time, those nine chapters were concerned with the mechanics of labor and delivery. However, the current understanding of labor includes a wide spectrum of preparedness even before the first regular contractions.

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Labor is the last few hours of human pregnancy. It is characterized by forceful and painful uterine contractions that effect cervical dilation and cause the fetus to descend through the birth canal. 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 this prolonged uterine quiescence, a transitional phase follows during which myometrial unresponsiveness is suspended and the cervix undergoes ripening, effacement, and loss of structural cohesion.

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The physiological processes that regulate parturition—the bringing forth of young—and the onset of labor continue to be defined. Three general contemporaneous 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. Current research supports a model that draws from all three themes. However, labor onset clearly 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, then preterm labor, dystocia, or postterm pregnancy may result. Of these, preterm labor remains the major contributor to neonatal mortality and morbidity.

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MATERNAL AND FETAL COMPARTMENTS

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Uterus
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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 can modulate the transition of the smooth muscle cell among phenotypes that provide cell growth, proliferation, secretion, and contractility.

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In addition to this phenotypic plasticity, 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 ...

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