[The onset of labour. A reminder of its physiology (author's transl)].

Until recently the onset of labour was considered to be due to changes in maternal hormone levels which were represented by a drop in the progesterone and a rise in the oxytocin levels. Recent work has shown that onset of labour is associated with several phenomena which take place parallel to one another, and they are immunological, haemodynamic, biochemical, mechanical, psychological and hormonal. The last of these are far and away the most important and have three sources: a placental source; a maternal source of sex steroids, oxytocin and prostaglandins and; of fetal origin, adrenal and posterior pituitary hormones. It seems that the prostaglandins and the maturation of the fetal brain are at present considered to be all-important. Better knowledge of the physiology of the onset of labour has led to greater success in pharmaco-dynamically induced labour, but this is not yet truly physiological. Therefore except in cases of pathology it does not seem right to undertake the risks of inducing labour more or less systematically.