Menstruation is the periodic discharge of blood and sloughed endometrium (collectively called menses or menstrual flow) from the uterus through the vagina. The rapid decline in ovarian production of progesterone and oestrogen that occurs each cycle in the absence of pregnancy causes menstruation.
Menstruation occurs throughout a woman’s reproductive life in the absence of pregnancy.
The average duration of menses is 5 (±2) days. Blood loss per cycle averages 30 ml (normal range, 13 to 80 ml) and is usually heavy on the 2nd day. A saturated pad or tampon absorbs 5 to 15 ml. Menstrual blood hardly clots (unless the bleeding is unusually heavy), probably because fibrinolysin and other factors inhibit clotting.
The median, menstrual cycle length is 28 days (usual range, about 25 to 36 days). Generally, variation is maximal, inter-menstrual intervals are longer in the years immediately after menarche and before menopause when ovulation occurs less regularly. The menstrual cycle begins and ends with the first day of menses (day 1).
We divide the menstrual cycle into phases, usually based on ovarian status – follicular, ovulatory, and luteal.
Access more information about the menstrual cycle via the Merck’s Manual.