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  • Labor is the physiologic process of childbirth defined by regular uterine contractions that result in effacement and dilatation of the cervix followed by expulsive efforts, leading to the delivery of the fetus and placenta.

  • Delivery is the reflexive and spontaneous action of bearing down to achieve expulsion of the fetus and placenta.


Parturition is the process of orchestrated movements of the fetus through the birth canal that is initiated by an integrated set of biochemical and biomechanical changes within the myometrium, decidua, and cervix. Prior to the initiation of labor, there are biochemical connective tissue changes in the cervix that precede the increase in myometrial activity that leads to myometrial contractility. The clinical diagnosis of labor is usually made retrospectively and is classically defined as regular painful uterine contractions resulting in changes in cervical effacement and dilation. As the fetus negotiates through the pelvis, there are complex interactions of 3 mechanical variables known as the “3 Ps”: power, passenger, and passage.

The Mechanics of Labor

Power refers to the force generated by uterine contractions. This activity can be qualitatively assessed by palpation of the uterine fundus or external tocodynamometry or quantitatively by internal manometry or pressure transducers, which measure contractions by Montevideo units. Classically, the presence of 3 to 5 contractions in 10 minutes has been used to define an adequate contraction pattern and, if accompanied by cervical change, then adequate labor. An alternative definition of adequate labor is contractions at a frequency and intensity that add up to 200 to 250 Montevideo units within a 10-minute window.

The passenger refers to the fetus, which has multiple factors that can affect its navigation through the pelvic canal. These variables include the following: fetal size or estimated fetal weight (EFW), fetal lie, presentation, and position and station in the pelvis. These fetal factors can be assessed by ultrasound and/or Leopold maneuvers.

The passage refers to the bony pelvic and soft tissue of the birth canal. There are 4 major pelvic types (Fig. 7–1): gynecoid, android, platypelloid, and anthropoid. The bony pelvis was previously measured by imaging studies such as x-ray, but current clinical practice is to evaluate the pelvis by clinical pelvimetry.

Figure 7–1.

Flexions of the fetal head in the 4 major pelvic types. (Reproduced with permission from Danforth DN, Ellis AH. Midforceps delivery: A vanishing art? Am J Obstet Gynecol 1963 May 1;86:29–37.)

During the third trimester of pregnancy, irregular, generally painless uterine contractions occur with slowly increasing frequency. These contractions, known as Braxton Hicks contractions, may occur more frequently, sometimes every 10–20 minutes, and with greater intensity during the last weeks of pregnancy. When these contractions occur in regular, frequent contraction prior to ...

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