Causes of onset of labor (1)
uterine distension: over-stretching of the uterus may play some part in onset of labor. Stretching effect on the myometrium by the growing size of the fetus and amniotic liquor can explain the onset of labor at least in twins or hydramnios. However “optimal distension theory” fails to account for the otherwise causeless preterm labor.
Feto-placental contribution: unknown factors stimulates fetal pituitary prior to onset of labor increased release of ACTH stimulates fetal adrenals increased cortisol secretion accelerated production of estrogen and prostaglandins from the placenta.