Labour or parturition is a physiologic process culminating in expulsion of fetus, amniotic fluid, placenta and membranes from the gravid uterus of a pregnant woman involving sequential and integrated changes in the myometrium, decidua and cervix. In a woman with a regular 28-day cycle, labour is said to take place 280 days after the onset of the last menstrual period. However, the length of human gestation varies considerably among healthy pregnancies, even when ovulation is accurately measured in naturally conceiving women. Initiation of labour may be best regarded as a withdrawal of the inhibitory effects of pregnancy on the tissue of the uterus, rather than as an active process mediated by the release of uterine stimulants. Successful labour passes through three stages: the shortening and dilatation of the cervix; descent and birth of the fetus; and the expulsion of the placenta and membranes. Efficient uterine contractions (power), an adequate roomy pelvis (passage) and an appropriate fetal size (passenger) are key factors in this process.