What hormone is primarily responsible for milk ejection during breastfeeding?

The hormone responsible for milk ejection during breastfeeding is oxytocin. Oxytocin is produced in the hypothalamus and released from the posterior pituitary gland in response to stimuli such as a baby suckling at the breast. This hormone triggers the contraction of myoepithelial cells surrounding the alveoli in the mammary glands, promoting the release of milk into the ducts, allowing it to be ejected through the nipple.

While other hormones play crucial roles in lactation, such as prolactin, which is primarily responsible for the production of milk, it is oxytocin that directly facilitates the ejection of milk during breastfeeding. Estrogen and progesterone also have significant roles in preparing the breast for lactation and maintaining pregnancy, but they do not directly cause the milk-ejection reflex. Thus, oxytocin's specific action in stimulating milk ejection distinguishes it as the key hormone in this process.