The physiology of labour

The time of onset of labour is usually reasonably clear-cut and will be described by the patient as the onset of painful uterine contractions. The exact reason whereby the uterus changes from a retentive to an expulsive organ is not yet clearly understood. It probably is the summation of several factors: the drop in the hormone progesterone, either in absolute terms or relative to the level of oestrogen; attainment of a critical degree of distension and intrauterine pressure; a critical change in the consistency or sphincteric quality in the cervix; a critical build up of contractile protein in the uterine muscle cells and its increasing sensitivity to oxytocic hormone(s).