It can take up to 3 years from the time a girl starts menstruating for her body to develop a regular cycle. Even then, what’s regular varies from person to person. Girls’ cycles can range from 21 to 45 days.

Changing hormone levels might make your period short one month (such as 2 or 3 days) and more drawn out (such as 7 days) the next. You might skip a few months, get two periods almost right after each other, have a really heavy period, or one so light you almost don’t notice it. (If you’re sexually active and you skip a period, though, you should visit your doctor or a women’s clinic to make sure you’re not pregnant.)

All this irregularity can make planning for your period a real hassle. Try to keep track of when your last period started, and guess that about 4 weeks from that day you could be due for another. If you’re worried about wearing that cute dress and suddenly starting your period at school, just make sure you pack protection. Carry a pad or tampon in your backpack, and wear a pantiliner to handle the first wave.

When it comes to periods, every girl’s body has a unique (and unpredictable) timeline for getting on track. If your period still has not settled into a relatively predictable pattern after 3 years, or if you have four or five regular periods and then skip your periods for a couple of months, make an appointment with your doctor to check for possible problems.