A Girl's First Period Dear BeingGirl Experts, I got my period last month on September 1, and I didn't get it this month (October). Is there something wrong or does it just come on a different day this month, or does it come like next month or something? Help? Rosanne --- Dear Rosanne, Having irregular periods isn’t unusual as a teen in the first few years after starting your period. You may not have one for several months or you may have another one in just a few weeks. You may start out with a regular cycle and then become irregular. Having an unpredictable cycle is not uncommon during your teen years. We recommend keeping a private menstrual calendar, and marking it when you have your period. Determine the length of your cycle is done by counting from the first day your period begins to the first day of your next period. Your period usually doesn't come on the same date each month. If you've already become regular, then each cycle is usually the same number of days; anything from 21 to 35 is average. However, for some girls the cycle alternates so that every other month the cycle is a different number of days. Periods average 2 –7 days. Hope this is helpful, Your BeingGirl Experts Questions about a girls first period? Get answers, tips, and advice at BeingGirl.com.