Pregnancy first - always - then the list sorts by circumstance, per OWH: breastfeeding, hormonal birth control (absent periods are expected on the shot, implant, hormonal IUD), perimenopause, low weight or hard training, stress, thyroid, PCOS.
Three or more missed months is amenorrhea, and that is the workup line - the causes matter beyond the period: low estrogen erodes bone. Sudden stop with hot flashes under 40: ask about primary ovarian insufficiency specifically.
What to do
Common questions
Why did my period stop?
Rule out pregnancy first - it is the commonest cause of a stopped period at every age it can happen, and tests are cheap and private. After that, the causes sort by life stage and circumstance, per the Office on Women's Health: breastfeeding (periods commonly stay away for months); hormonal birth control, especially the shot, implant, and hormonal IUD, where absent periods are expected; perimenopause - cycles space out before stopping, typically in the 40s; low body weight, eating disorders, and hard athletic training (the body's "not safe for pregnancy" switch); significant stress; thyroid problems; PCOS; and, rarely, pituitary causes. The medical name for three or more missed months is amenorrhea, and per OWH that is the workup line: worth a visit for the hormone panel - pregnancy test, thyroid, prolactin - because the underlying causes matter beyond the period itself (bone health suffers when estrogen runs low). Sudden stop with hot flashes under 40: ask about primary ovarian insufficiency specifically. Pymander, a free AI doctor available 24/7 by text, can help you prep for the visit.