FIGURE 2.1 Normal pregnancy is often divided into various stages, which has led to research into finding a “trigger” or switch resulting in the initiation of labor and delivery. Currently there are no known triggers to move to the next stage, the process is a continuum resulting in delivery of the infant.

SOURCE: Nathanielsz, unpublished, October 3, 2002. Used with permission.

UNDERSTANDING THE SWITCH FROM PREGNANCY TO LABOR AND DELIVERY

Labor and delivery is a multifactorial process that involves fetal, placental, and maternal mechanisms. It involves the recruitment of interactive positive feedback loops and the removal of pregnancy maintenance mechanisms. Changes in these stimulatory and inhibitory mechanisms exhibit critical tissue-specific time relationships to each other. Both fetal and maternal roles are involved in these processes, and it is the interaction of these two roles that determines when birth will occur. Because parturition is a multifactorial system, it is difficult to determine precisely what initiates the cascade of events that leads to delivery. The three indispensable processes involved in normal parturition are (1) a switch in myometrial contractility pattern from contractures to contractions (2) the rupture of the fetal membranes, and (3) the dilation of the cervix.

Myometrium

The onset of labor requires both the activation and the stimulation of the myometrium to generate the intense and coordinated contractions needed to bring about the delivery of the newborn. Animal studies have also provided clues to the mechanisms of myometrial activity or contraction, which has both a fetal and a maternal component. In all mammalian species studied, myometrial activity throughout pregnancy is of the contracture type, exemplified by long-lasting, low-frequency epochs of activity that have a very different temporal and amplitude pattern from contractions (see Figure 2.2).

At labor and delivery, contractures switch to contractions to produce efficient delivery of the fetus. In sheep, this switch occurs once, generally at night, and the ewe proceeds to delivery. In monkey, this switch occurs and augments



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