A regular menstrual cycle starts 28 days after the previous cycle began. The first day of a menstrual cycle is considered to begin on the first day of regular blood flow.
Menstrual cycles of young teens can be irregular lasting from 21 to 45 days, while the cycles of adults can last from 21-35 days. In general, the first two years of menstruation produce the longest, most irregular cycles. Over time, menstrual cycles normalize and occur after a set amount of time; although, many women never have a 28-day cycle. Birth control pills, weight loss, exercise and ovary disorders can cause irregular cycles.