What is the menstrual cycle?

Menstruation is part of the menstrual cycle, which prepares your body for pregnancy each month. A cycle is counted from the first day of one period to the first day of the next period. The first day of your cycle is the first day of blood flow.  The average menstrual cycle is 28 days long. Cycles can range anywhere from 21 to 35 days in adults and from 21 to 45 days in young teens.

Body chemicals called hormones rise and fall during the month to make the menstrual cycle happen.