What is FSH and how is it regulated?

Follicle-Stimulating Hormone, or FSH, is the hormone released by the pituitary gland in the brain that tells the ovary it is time to mature eggs, thus producing estrogen, and get the eggs ready for ovulation.

Each month after puberty, we have a cycle that is regulated by hormone signals released from our brain that tell the ovary to make female hormones: estrogen and progesterone. When estrogen is low, the brain sends FSH signals to the ovary to mature eggs, so more estrogen is made. Since estrogen is produced by eggs -- the more eggs you have, the more estrogen your ovaries can make. 
As we lose eggs or our egg quality declines with age, the harder it is for the ovary to make estrogen. As a result, the brain must send more FSH to try and get any estrogen produced.