At first, your periods may be irregular. They may not come every month (roughly 28 days), but more often (every 3 weeks) or less often (every 6 weeks). Don’t worry, this is normal. It can take up to three years for a woman's period to become regular.
Remember every woman's menstrual cycle is different. A way to figure out what is a regular menstrual cycle for you, is to jot down the first day and last day of your period on a calendar for a few months.
What if your periods haven't started by the time you're 16? There could be many reasons why your periods are delayed. It could be because of diet, stress, exercise, or pregnancy. The best thing to do is contact your nearest health care professional to make sure everything is OK.