19. Birth
Towards the end of pregnancy progesterone receptors are blocked so progesterone is no longer effective
As a result, the pituitary gland will produce the hormone oxytocin
Stretching of the lower uterus walls by the fetus will add to the stimulus for the pituitary to secrete oxytocin
Oxytocin causes the smooth muscles in the uterine wall to contract (Labor)
The fact that oxytocin causes the uterine walls to be stretched which in turns stimulates the production of even more oxytocin is an example of positive feedback