Premenstrual syndrome (PMS) is the term for the physical and emotional symptoms that many girls and women get right before their periods begin each month. Doctors have not pinpointed the exact cause of PMS, but it seems to be linked to changing hormone levels.

Cramps
Lots of?women have abdominal cramps during the first few days of their periods. Cramps are most likely caused by prostaglandins, chemicals your body produces that make the muscles of the uterus contract.

Irregularity
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.) 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 22 to 45 days.

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.