FIGURE 6-1. The stages of parturition.


The stages of parturition. Following implantation, more than 95 percent of gestation is spent in Phase 0, uterine quiescence. During quiescence, myometrial contractility is inhibited by a variety of biological substances, including progesterone. Phase 1, myometrial activation, is characterized by increased expression of contraction-associated proteins, receptors for oxytocin, and prostaglandins, and increased placental estrogen biosynthesis. The signal for myometrial activation is controlled by the fetal HPA axis, which, in turn, is up-regulated by endogenous placental CRH production. Phase 2, myometrial stimulation, involves a progressive cascade of events, beginning with myometrial activation, which results in myometrial contractility, cervical ripening, and decidua and membrane activation. It is likely initiated by the same events of fetal HPA activation that initiate Phase 1. Phase 3, involution, involves placental separation and contraction of the uterus. It is primarily effected by maternal oxytocin. Implantation

From: 6, Biological Pathways Leading to Preterm Birth

Cover of Preterm Birth
Preterm Birth: Causes, Consequences, and Prevention.
Institute of Medicine (US) Committee on Understanding Premature Birth and Assuring Healthy Outcomes; Behrman RE, Butler AS, editors.
Washington (DC): National Academies Press (US); 2007.
Copyright © 2007, National Academy of Sciences.

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