Labor is the physiological process by which a fetus is expelled from the uterus to the outside world. This topic will discuss the process of normal labor and delivery. Management of labor and delivery is reviewed separately. (See "Management of normal labor and delivery".)
Labor is a physiological event involving a sequential, integrated set of changes within the myometrium, decidua, and uterine cervix that occurs sometimes gradually over a period of days to weeks and sometimes rapidly over minutes to hours and culminates in delivery of the fetus. Biochemical connective tissue changes in the uterine cervix appear to precede uterine contractions and cervical dilation, and all of these events usually occur before rupture of the fetal membranes. These physiological changes are discussed in detail elsewhere. (See "Physiology of parturition".)
Uterine contractions during active labor have two major functions: to dilate the cervix and to push the fetus through the birth canal. However, the fetus is not merely the passive recipient of these forces, rather, its ability to successfully negotiate the pelvis is dependent upon the complex interaction of three mechanical variables, known as the "three Ps": the powers, the passenger, and the passage.
The effect of maternal obesity on labor is discussed separately. (See "The impact of obesity on female fertility and pregnancy".)
Powers (uterine contractions) — Powers refer to the force generated by the uterine musculature during contractions. It is generally believed that the more optimal the powers, the more likely a successful outcome; however, there are no data to support this statement.