This structure becomes a center for the synthesis of progesterone, a molecule that both inhibits pituitary LH production, and promotes the growth of the uterus. In the absence of fertilization, the corpus luteum involutes, leading to a period of menses with low circulating estrogen and progesterone levels. The lack of these two hormones is a positive signal for new FSH release and the start of a new cycle. Although estrogen is the primary stimulus for the "LH spike", gonad-produced inhibin is believed to limit the amount of FSH released during this spike, thus fine-tuning the response. In addition, during the luteal or post-ovulation period, ovarian-derived inhibin levels rise, helping to suppress pituitary FSH release during this time. Then when inhibin levels fall late in the menstrual cycle, an impediment to FSH production is removed.