Which hormone is responsible for the letdown reflex that allows ejection of breast milk?

The hormone responsible for the letdown reflex that allows for the ejection of breast milk is oxytocin. During breastfeeding, when a baby suckles at the breast, nerve impulses travel to the mother's brain, prompting the release of oxytocin from the posterior pituitary gland. This hormone causes the muscles around the milk ducts in the breast to contract, resulting in the ejection of milk into the nipple, making it available to the nursing infant.

In contrast, while estrogen and progesterone play significant roles in breast development and prepare the mammary glands for milk production during pregnancy, they are not directly involved in the ejection reflex. Prolactin is crucial for milk production, stimulating the mammary glands to produce milk, but it does not facilitate the actual ejection of that milk. Therefore, understanding these roles highlights why oxytocin is key to the letdown reflex, ensuring that the process of breastfeeding can effectively occur.