During the follicular phase of the menstrual cycle, about five to seven follicles in your ovary (and sometimes both ovaries) start racing towards the finish line. Their growth is encouraged by the FSH hormone. The name gives it away –- FSH is follicle-stimulating hormone, or in other words, the hormone which stimulates the follicles to grow.
As the follicles grow larger, they begin to release the hormone estrogen. As this estrogen travels through the bloodstream, it makes its way back to the pituitary gland, causing the gland to decrease the FSH production. This is called negative-feedback – as estrogen rises, FSH lowers.
However, this negative-feedback cycle switches to a positive-feedback cycle as the follicle reaches the final stages of maturity. Eventually, one (and sometimes two) of the follicles become a dominant follicle. The dominant follicle releases an even greater amount of estrogen into the bloodstream.
When the estrogen levels reach a certain threshold, estrogen suddenly switches to having a positive-feedback effect on FSH. In other words, the high levels of estrogen suddenly cause a spike in FSH, kind of like a last jolt to the maturing egg.
But after this last FSH sprint, the pituitary gland abruptly slows down the production of FSH. This is the beginning of the next phase of the menstrual cycle, known as the ovulatory phase.