During the first 11 days of the cycle, estrogen exerts negative feedback control over the secretion of LH and FSH. This decrease in FSH causes the regression of all the follicles except the emerging dominant follicle. Eventually, however, the estrogen levels increase to such a degree that the mode of control regarding LH shifts from negative feedback to positive feedback. In other words, estrogen increases rapidly in the late follicular phase and triggers the LH surge.

Hormonal control of the menstrual cycle is rarely understood from the first source. This is a topic for which learning is really helped by getting a number of narrations.