What triggers a period to begin?
Short answer: A period begins when progesterone and estrogen levels fall at the end of the menstrual cycle, causing the uterine lining (endometrium) to break down and be shed. This hormonal drop follows the degeneration of the corpus luteum when fertilization does not occur and is coordinated by the hypothalamus–pituitary–ovarian axis.
Explanation
The menstrual cycle is regulated by signals from the hypothalamus and pituitary that control the ovaries. After ovulation the corpus luteum produces progesterone and some estrogen to maintain the endometrium. If no pregnancy occurs, the corpus luteum degenerates about 10–16 days after ovulation, causing a rapid decline in progesterone (and usually estrogen). The hormone withdrawal makes the endometrial blood vessels constrict and then break down; prostaglandins increase, causing uterine contractions that help expel tissue and blood. Cycle length and timing vary between individuals; illness, stress, body weight changes, medications or hormonal contraceptives can alter hormone patterns and the timing of bleeding.
Tips
- Track cycle length and symptoms to recognize your typical pattern.
- Seek medical advice if periods are very heavy, extremely painful, absent for months, or highly irregular.
- Know that stress, sudden weight loss/gain, intense exercise, and some medications can delay or skip periods.
- Hormonal contraception and some medical conditions (e.g., PCOS, thyroid disorders) change hormone signals and bleeding patterns.
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