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