Women with irregular periods usually blame stress or other lifestyle factors, and often use hormonal birth control to make their cycle regular. But an irregular period could signal something serious: primary ovarian insufficiency, characterized by a lack of reproductive hormones. The condition affects 1 in 100 women by age 40, and it greatly reduces fertility, NPR reports.
Birth control pills can be a dangerous deception to sufferers of POI, because the external hormones regularize periods. After starting on the Pill, a woman may think, "'Oh, it looks like things are fine now, because my periods are coming,'" said a POI researcher. "But, in fact, their ovaries aren't supplying the hormones to make that happen, so it's masking the fact that their ovaries aren't working."