Signals of parturition originate in the form of mild uterine contraction. The foetus signals that is ready (mature) by secreting certain hormones which diffuse through placenta into mother's blood and cause secretion of oxytocin which initiate uterine contractions. These are called foetal ejection reflexes. It is accompanied by rise in oestrogen to progesterone ratio, increase in oxytocin receptors in uterine muscles, increase in level of oxytocin secretion by mother's pituitary gland.
Related Theory
As onset of labour cause complete dilation of cervix, uterine contractions begin from top. They force the baby towards cervix. Contractions are accompanied by pain caused by compression of blood. These confractions are under positive feedback. Oxytocin induce uterine contractions which in turn caused increased oxytocin secretion. Relaxin hormone produced by the placenta increases flexibility of pubic symphysis and dilates uterine cavity which helps in pushing the baby down into the cervix. This results in rupturing of amniotic membrane and flowing out of amniotic fluid.