Phases of labor

It is useful to consider labor as a series of four physiologic phases, characterized by the release of the myometrium from the inhibitory effects of pregnancy and the activation of stimulants of myometrial contractility (Fig. 22.1). Phase 0 comprises the majority of pregnancy. During this phase, the uterus is maintained in a state of quiescence by one or more inhibitors of contractility. Candidate inhibitors include progesterone, prostacyclin, nitric oxide, parathyroid hormone-related peptide (PTHrP), calcitonin gene-related peptide, relaxin, adrenomedullin and vasoactive intestinal peptide (VIP). Near the end of a normal pregnancy, the uterus undergoes the process of activation (Phase 1). During activation, a number of contraction-associated proteins increase under the influence of estrogen. These proteins include myometrial receptors for prostaglandins and oxytocin, membranous ion channels and connexin-43, a key component of gap junctions. The increase in myometrial gap junctions during activation will electrically couple adjacent myometrial cells and maximize the coordination of contraction waves that move from the uterine fundus to the cervix. Phase 2 of labor is called stimulation. During stimulation, oxytocin and stimulatory prostaglandins (PGs) such as PGE₂ and PGF_2α can induce contractions in the previously primed uterus. The cervix dilates. The fetus, membranes and placenta are expelled from the uterus in a process called parturition. Phase 3 of labor follows parturition and is called involution. During involution, sustained contraction of the uterus promotes necessary hemostasis and eventually reduces the massively enlarged postpartum uterus to a size only slightly larger than its prepregnant state.