Which hormone is primarily responsible for stimulating milk secretion in new mothers?

Prolactin is the hormone primarily responsible for stimulating milk secretion in new mothers. It is secreted by the anterior pituitary gland and plays a crucial role during and after pregnancy. Prolactin levels increase significantly during pregnancy, preparing the mammary glands for lactation. Following childbirth, the decreased levels of estrogen and progesterone allow prolactin to facilitate milk production in response to the baby's suckling. This process not only promotes milk synthesis but also ensures an adequate supply of breast milk for the infant.

In contrast, estrogen is involved in the development of breast tissue during pregnancy but does not directly stimulate milk secretion. Oxytocin, while important for milk ejection during breastfeeding, primarily facilitates the release of already produced milk rather than its secretion. Progesterone also aids in preparing the breast for milk production and plays a role in maintaining pregnancy but does not stimulate secretion. Therefore, prolactin's direct function in milk production makes it the key hormone in this process.