Cycle basics · 4 minute read
What happens during the menstrual phase?
Why bleeding starts, what the uterus is shedding, and which changes are useful to log.
Evidence-informed · Editorial review August 2026 · Links to trusted sources are available in the interactive app
Why a period begins
If pregnancy has not occurred, estrogen and progesterone fall. That hormonal shift triggers the uterus to release its built-up lining. Blood and tissue leave through the cervix and vagina.
Flow varies across the period
Many people have heavier flow early and lighter flow later, but patterns differ. Track start and end dates, heaviness, clots, pain, and whether symptoms interfere with sleep, school, work, or daily activity.
When the pattern deserves attention
Contact a clinician about bleeding that is much heavier or longer than usual, cycles repeatedly outside your normal range, bleeding between periods, or pain that limits daily life. Seek urgent care for severe symptoms such as fainting, chest pain, or soaking products very rapidly.