Final answer:
Oxytocin, produced by the posterior pituitary, stimulates uterine contractions during childbirth.
Step-by-step explanation:
Oxytocin, a hormone produced by the posterior pituitary, stimulates uterine contractions during childbirth. Although the smooth muscle cells of the uterus are not initially sensitive to oxytocin, the number of oxytocin receptors in the uterus increases towards the end of pregnancy. This sensitivity increase is due to the expression of more receptors by the myometrium. The feedback relay between the uterus, hypothalamus, and posterior pituitary ensures an adequate supply of oxytocin, leading to stronger contractions as labor progresses.