what is the menstrual cycle? The menstrual cycle is a monthly process that prepares the body for pregnancy. It starts on the first day of your period and ends on the day before your next period starts. The average menstrual cycle is 28 days long, but it can vary from woman to woman. 49% of our population is female and 25% of the population bleeds but that 49% all have the same female biochemistry of hormones. Meaning higher estrogen and progesterone (along with some other minor hormones) than male counterparts.