Periods (menstruation) happen because the body is preparing for a possible pregnancy each month; when pregnancy does not occur, the uterus sheds its built‑up lining and that blood and tissue leave the body through the vagina. This monthly shedding is a normal part of the reproductive cycle for people with a uterus, usually from puberty until menopause.

What a period actually is

  • Each cycle, hormones tell the uterus to grow a thick, blood‑rich lining where a fertilized egg could implant and grow.
  • If no egg is fertilized, those hormone levels drop and the lining breaks down and leaves the body as menstrual blood (a period).
  • The cycle then starts again, creating a repeating pattern (a periodic cycle), which is why it is often called a “menstrual cycle.”

Why the body “bothers” with periods

  • Periods are the body’s way of resetting the uterus to keep it ready for a future pregnancy while also clearing out tissue that is no longer needed.
  • Regular cycles can be a sign that hormones, the ovaries, and the uterus are working together as they should, even though symptoms like cramps or mood changes can be uncomfortable.
  • From an evolutionary point of view, menstruation is one strategy some species use to manage and renew the uterine lining rather than keeping it indefinitely.

Why it’s called a “period”

  • The word “period” comes from the Greek “periodos,” meaning “a recurring cycle or path,” which later became the Latin “periodus” and then the English term.
  • The name stuck because menstrual bleeding usually happens at regular, repeating intervals, like a cycle that comes around again and again.

TL;DR: We have periods because the uterus builds a lining each cycle in case of pregnancy, and when pregnancy doesn’t happen, that lining is shed as blood and tissue in a repeating monthly cycle.