Normal labor: Mechanism and duration

Labor refers to the chain of physiologic events that allows a fetus to undertake its journey from the uterus to the outside world. The mean duration of a singleton pregnancy is 40.0 weeks (280 days), which is dated from the first day of the last normal menstrual period. The period from 37.0 weeks (259 days) to 42.0 weeks (294 days) of gestation is regarded as term. This article focuses on the onset, progress, and mechanics of normal labor at term. Topics such as preterm labor (labor before 37 weeks), postterm labor (labor after 42 weeks), and abnormal labor and delivery have not been addressed and are discussed in detail elsewhere in this issue.