Which hormone is primarily responsible for the growth of breast lobules and ducts during the menstrual cycle?

The hormone primarily responsible for the growth of breast lobules and ducts during the menstrual cycle is estrogen. During the follicular phase of the cycle, rising levels of estrogen lead to the proliferation of ductal epithelium and the growth of lobular structures in the breast tissue. This preparation is vital for potential lactation in the event of pregnancy.

Estrogen stimulates the development of the breast tissue, but progesterone also plays a critical role, particularly in preparing the breast for lactation and influencing the secretion of milk. However, the initial growth and development of ducts and lobules are predominantly influenced by estrogen's actions during the menstrual cycle.

Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) are involved in regulating the menstrual cycle and ovarian function but do not directly promote the growth of breast tissues. Thus, while all these hormones have important roles in reproductive health, estrogen is specifically the primary hormone for breast development during the menstrual cycle.