Causes Of Irregular Periods Dear BeingGirl Experts, I am a bit embarrassed to talk to my mom about this so: How come I might get my period every 20 days, and then not get it again for like 30? Melissa --- Dear Melissa, First periods are often very irregular. Keeping a private calendar and marking it when you have your period is a smart idea. This will help you keep up with your period. You count from the first day of your period to the first day of your next period to determine how many days your cycle is. Your period probably won’t come on the same date each month. Teen girls are often a surprised by this. You may find your menstrual cycle lasts 28 days, others might have a 24-day cycle, a 30-day cycle, or even longer. After you start your first period, menstrual cycles last 21–45 days. After a few years, cycles usually shorten to an adult length of 21–34 days and become more regular. Periods average 3–7 days. It often takes several years for a predictable cycle to develop, so be understanding of the time this requires. Your mother is your resident expert, and can help you better understand what to expect if you ask. Hope this is helpful, Your BeingGirl Experts Read about the causes of irregular periods at BeingGirl.com. get expert tips and advice.