Menstruation is a woman’s monthly bleeding, often called your “period.” When you menstruate, your body discards the monthly buildup of the lining of your uterus (womb). Menstrual blood and tissue flow from your uterus through the small opening in your cervix and pass out of your body through your vagina.

During the monthly menstrual cycle, the uterus lining builds up to prepare for pregnancy. If you do not get pregnant, estrogen and progesterone hormone levels begin falling. Very low levels of estrogen and progesterone tell your body to begin menstruation.

What is the menstrual cycle?

The menstrual cycle is the monthly hormonal cycle a female’s body goes through to prepare for pregnancy. Your menstrual cycle is counted from the first day of your period up to the first day of your next period. Your hormone levels (estrogen and progesterone) usually change throughout the menstrual cycle and can cause menstrual symptoms.

The typical menstrual cycle is 28 days long, but each woman is different.2 Also, a woman’s menstrual cycle length might be different from month-to-month. Your periods are still “regular” if they usually come every 24 to 38 days. This means that the time from the first day of your last period up to the start of your next period is at least 24 days but not more than 38 days.

Some women’s periods are so regular that they can predict the day and time that their periods will start. Other women are regular but can only predict the start of their period within a few days.