how often is period
The last day before your next period would be Day 28 if you have the average 28-day menstrual cycle. Normal menstrual cycle length, in adolescent girls, is 21 to 45 days. Normal menstrual cycle length in adult women is from 21 to 35 days. During the first few years of menstruation, periods are often irregular. You may skip several months between your first period, or menarche, and the next time you menstruate. It is also probable that you may have two periods that are 28 days apart and then skip a period.