Labour is a physiological process during which the foetus, membranes, umbilical cord and placenta are expelled from the uterus
Physiology
Initiation of labour
Change in estrogen/progesterone ratio
Fetal adrenals and pituitary hormones may control timing of the onset of labour
Myometrial stretch increases excitability of myometrial fibres
Mechanical stretch of cervix and fetal of membranes
Ferguson's reflex
Hormonal factors influencing the onset of labour
Progesterone keeps the uterus settled - prevents formation of gap junctions, hinders the contractibility of myocytes
Estrogen makes the uterus contract and promotes prostaglandin production
Oxytocin initiates and sustains contraction, acts on decidual tissue to promote prostaglandin release
Oxytocin is synthesized directly in decidual and extraembryonic fetal tissues, and in the placenta
Other causes
Pulmonary surfactant secreted into amniotic fluid has been reported to stimulate prostaglandin synthesis
Increase in production of fetal cortisol stimulates an increase in maternal estriol
Increase in myometrial oxytocin receptors and their activation results in phospholipase C activity and subsequent increase in cytosolitic calcium and uterine contractility
Cervical changes
Cervical softening - increase in hyaluronic acid will decrease bridging among collagen fibres, decreasing firmness of the cervix
Cervical ripening - changes include a decrease in collagen fibre alignment and strength, decrease in tensile strenght of the cervical matrix, and an increase in cervical decorin