Definition
Labor is the physiologic process by which a fetus is expelled from the uterus to the outside world. It is a clinical diagnosis requiring two elements: (1) regular phasic uterine contractions increasing in frequency and intensity, and (2) progressive effacement and dilation of the cervix. Normal labor occurs at term (defined as 37–0/7 to 42–0/7 weeks’ gestation).
The endocrine control of labor
Labor may be regarded physiologically as a release from the inhibitory effects of pregnancy on the myometrium rather than as an active process mediated by uterine stimulants. In vivo, however, both inhibitory and stimulatory factors appear to be important. It is likely that there is a “parturition cascade” at term that removes the mechanisms maintaining uterine quiescence and recruits factors promoting uterine activity (see Chapter 37). Regardless of whether the trigger for labor begins within the fetus or outside the fetus, the final common pathway ends in the maternal tissues of the uterus, and is characterized by the development of regular phasic uterine contractions.
Myometrial contractility
As in other smooth muscles, myometrial contractions are mediated through the ATP-dependent binding of thick filaments (myosin) to thin filaments (actin) (Figure 60.1
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