If fertilization does not occur and menstruation happens, inhibin and progesterone levels drop, leading to the secretion of GnRH, FSH, and LH, initiating a new follicular phase.

“And if menstruation occurs, well, then inhibin levels also drop. Progesterone levels also drop. And when that occurs, there's a positive feedback signal up at the level of the pituitary. The pituitary literally can register how much inhibin and progesterone and estrogen is present in the bloodstream. And if those levels are sufficiently low, well, then GnRH gets secreted again, FSH gets secreted again, and LH gets secreted again. And the first half the follicular phase of the menstrual cycle initiates all over again.”