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Physiology of parturition

Author
Errol R Norwitz, MD, PhD, MBA
Section Editor
Charles J Lockwood, MD, MHCM
Deputy Editor
Vanessa A Barss, MD, FACOG

INTRODUCTION

Labor is a physiological event involving a sequential, integrated set of changes within the myometrium, decidua, and uterine cervix that occur gradually over a period of days to weeks. 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.

Labor is a clinical diagnosis, which includes (i) the presence of regular phasic uterine contractions increasing in frequency and intensity, and (ii) progressive cervical effacement and dilatation. A show (bloody discharge) may or may not be present. Cervical dilatation in the absence of uterine contractions is seen most commonly in the second trimester and is suggestive of cervical insufficiency (see "Cervical insufficiency"). Similarly, the presence of uterine contractions in the absence of cervical change does not meet criteria for the diagnosis of labor. Such contractions are often attributed to "false labor" or uterine irritability. The myometrial contractility pattern changes in labor from "contractures" (long-lasting, low frequency activity) to "contractions" (high intensity, high frequency activity) [1].

Labor and delivery are not passive processes by which uterine contractions push a rigid object through a fixed aperture. The ability of the fetus to successfully negotiate the pelvis during labor and delivery depends upon a complex interaction of three variables: power (uterine contractions), passenger (fetus), and passage (both bony pelvis and pelvic soft tissues). Although conventional wisdom dictates that powerful contractions are more likely to be associated with a successful outcome, there are no data to support this conclusion. Furthermore, precipitous labor probably results from low resistance of the pelvic soft tissues (the cervix in the first stage of labor and the muscles of the pelvic floor in the second stage) rather than from high myometrial activity [2].

LABOR AT TERM

The mean duration of human singleton pregnancy is 280 days (40 weeks) from the first day of the last menstrual period. A term pregnancy is defined as the period from 259 to 293 days after the first day of the last menstrual period (370/7ths to 416/7ths weeks).

Term 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. As an example, strips of myometrium obtained from a quiescent uterus at term and placed in an isotonic water bath will contract vigorously and spontaneously without added stimuli [3,4]. Nevertheless, both inhibitory and stimulatory mechanisms likely play a role in uterine activity.

                       

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Literature review current through: Nov 2016. | This topic last updated: Tue Jun 16 00:00:00 GMT+00:00 2015.
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