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is rather uncommon in physiology. But it is not uncommon for hormones to have different effects depending on whether they are present for a short time or for a long time.

So, in a fertile woman, estrogen levels that have been elevated for a short time make the hypothalamus stop secreting GnRH; but if those estrogen levels remain high for a longer time, they signal to the hypothalamus that one of the follicles has survived and matured, and then the hypothalamus starts secreting GnRH again.

If the hypothalamus didn't start up again, the woman's ovaries would never be able to release the egg. She would be infertile.

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