The monthly interplay of hormones begins with a surge of GnRH, or Gonadotropin Releasing Hormone (not shown). This triggers a burst of Follicle Stimulating Hormone (FSH) and a gradual increase in Luteinizing Hormone (LH). Responding to the FSH, a dominant follicle matures, releasing a surge of estrogen. This crescendo in estrogen levels prompts a burst of LH, which stimulates the remnants of the dominant follicle, now called the corpus luteum, to flood the system with progesterone. If conception doesn't occur, the system is self-limiting. Rising progesterone prompts a decline in LH; and less LH means less progesterone. When estrogen and progesterone levels hit bottom, it's the signal for a new surge of GnRH, starting the whole cycle anew.