The two major steroid hormones produced by the human ovaries are progresterone and estradiol, a form of estrogen. At this point, rising levels of FSH stimulate the development of several follicles in the ovaries. These early follicles in turn begin to produce estrogen. Usually, only one of these follicles will develop past the early stage. Estrogen levels continue to rise as the follicle develops.
Progesterone eventually begin to rise as well.
Up to a certain point, the estrogen being produced exerts negative feedback on both GnRH and gonadotropin secretion. Around the 14th day of the cycle, the anterior pituitary abruptly changes its response to the persistently high estrogen levels. Rather than suppressing gonadotropin release, the estrogen now has a positive feedback effect. The reasons for this switch are still not clear. High LH levels in turn initiates the process of ovulation, whereby the follicle ruptures and egg is released.