What initiates the follicular phase of the menstrual cycle?

The follicular phase of the menstrual cycle is initiated by a rise in follicle-stimulating hormone (FSH). In the early part of the cycle, the levels of progesterone decrease, which leads to the shedding of the uterine lining and the start of menstruation. As menstruation ceases, the anterior pituitary gland increases the secretion of FSH, which stimulates the growth and maturation of ovarian follicles. Each follicle houses an oocyte, and as these follicles develop, they produce estrogen, which further supports the growth of the endometrium in preparation for a potential pregnancy.

While a surge in luteinizing hormone (LH) and a rise in estrogen are also important in the menstrual cycle, they occur later. The LH surge is primarily associated with the transition from the follicular phase to ovulation, while estrogen levels rise as a result of follicular development prompted by FSH. Therefore, FSH is the critical hormone that initiates the follicular phase and is essential for the growth of the ovarian follicles.