Final answer:
The hormone oxytocin, produced in the hypothalamus and released by the posterior pituitary gland, causes uterine contractions and the milk ejection reflex from the breasts.
Step-by-step explanation:
The hormone responsible for the contractions of the uterus during labor and milk release from the breasts is called oxytocin. It is produced in the hypothalamus and released by the posterior pituitary gland. During childbirth, this hormone stimulates uterine smooth muscle contractions, and these contractions become more intense as the concentration of oxytocin in the blood increases through a positive feedback mechanism. Additionally, oxytocin is responsible for the milk ejection reflex, also known as "let-down," which occurs when the baby suckles at the breast, stimulating the contraction of myoepithelial cells around the mammary glands, resulting in milk being ejected from the breasts.