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Mechanism of normal labor and delivery

Edmund F Funai, MD
Errol R Norwitz, MD, PhD
Section Editor
Charles J Lockwood, MD, MHCM
Deputy Editor
Vanessa A Barss, MD, FACOG


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.


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Literature review current through: Oct 2015. | This topic last updated: Feb 26, 2014.
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  1. Chua S, Kurup A, Arulkumaran S, Ratnam SS. Augmentation of labor: does internal tocography result in better obstetric outcome than external tocography? Obstet Gynecol 1990; 76:164.
  2. CALDEYRO-BARCIA R, SICA-BLANCO Y, POSEIRO JJ, et al. A quantitative study of the action of synthetic oxytocin on the pregnant human uterus. J Pharmacol Exp Ther 1957; 121:18.
  3. Hauth JC, Hankins GD, Gilstrap LC 3rd, et al. Uterine contraction pressures with oxytocin induction/augmentation. Obstet Gynecol 1986; 68:305.
  4. Pattinson RC. Pelvimetry for fetal cephalic presentations at term. Cochrane Database Syst Rev 2000; :CD000161.
  5. FRIEDMAN E. The graphic analysis of labor. Am J Obstet Gynecol 1954; 68:1568.
  6. FRIEDMAN EA. Primigravid labor; a graphicostatistical analysis. Obstet Gynecol 1955; 6:567.
  7. Peisner DB, Rosen MG. Transition from latent to active labor. Obstet Gynecol 1986; 68:448.
  8. Duignan NM, Studd JW, Hughes AO. Characteristics of normal labour in different racial groups. Br J Obstet Gynaecol 1975; 82:593.
  9. Studd J. Partograms and nomograms of cervical dilatation in management of primigravid labour. Br Med J 1973; 4:451.
  10. Albers LL. The duration of labor in healthy women. J Perinatol 1999; 19:114.
  11. Zhang J, Troendle JF, Yancey MK. Reassessing the labor curve in nulliparous women. Am J Obstet Gynecol 2002; 187:824.
  12. Cesario SK. Reevaluation of Friedman's Labor Curve: a pilot study. J Obstet Gynecol Neonatal Nurs 2004; 33:713.
  13. Zhang J, Troendle J, Mikolajczyk R, et al. The natural history of the normal first stage of labor. Obstet Gynecol 2010; 115:705.
  14. Greenberg, M, Caughey, AB, Hopkins, LM, Stotland, NE, et al. Does the length of labor vary by ethnicity? Am J Obstet Gynecol 2005; 193:S41.
  15. Diegmann EK, Andrews CM, Niemczura CA. The length of the second stage of labor in uncomplicated, nulliparous African American and Puerto Rican women. J Midwifery Womens Health 2000; 45:67.
  16. World Health Organization. Maternal and Child Health and Family Planning. The prevention and management of postpartum haemorrhage. Report of a technical working group. WHO/MCH 1990; 90.7:3.