Within two years after you start menstruating.
The discharge of blood and tissue from the lining of your uterus is your menstrual period (also called your period). Most girls start their periods between the ages of 12 years and 14 years, but some start earlier or later. When you first start having your period, it may last only a few days. Your first few periods may be very light. You may only see a few spots of reddish brown blood. Anywhere from 2 to 7 days is normal. The average menstrual cycle is about 28 days, but cycles that are 21–45 days also are normal. It may take 6 years or more after your period starts for your cycle to get regular. If you do this every month, you may notice a pattern. It may become easier to tell when you will get your next period Don't worry if your early periods have longer cycles or don't follow a schedule. This irregularity is normal for at least the first 2 years. Your periods should become more regular within two years after you start menstruating. Some teens have a 28-day cycle; some have a 24-day cycle; others have a 30- to 34-day cycle. All of these are normal. For young teens, cycles can range from 21 to 45 days. For adults, it can be 21-35 days. If your period is much shorter or longer, or if your period does not become regular after two years, see your primary health care provider. Amenorrhea means not having a period. It is normal for some girls not to start their periods until age 16 years. However, your doctor may want to see you if you have not started by age 15 years. You also should see your doctor if you have started your period but it then stops for more than 3 months. The average menstrual cycle is about 28 days, but cycles that are 21–45 days also are normal. It may take 6 years or more after your period starts for your cycle to get regular. If you do this every month, you may notice a pattern. It may become easier to tell when you will get your next period But, anywhere from 2 to 7 days is normal. For the first few years after menstruation begins, longer cycles are common. A woman's cycle tends to shorten and become more regular with age. Most of the time, periods will be in the range of 21 to 35 days apart. A cycle is counted from the first day of 1 period to the first day of the next period. The average menstrual cycle is 28 days long. Cycles can range anywhere from 21 to 35 days in adults and from 21 to 45 days in young teens. 1 Age. 2  When teens first start having periods, their menstrual cycles may not always be on the same schedule every month. 3  It may take several years to settle into a pattern. 4  In addition, missed periods and lighter or heavier periods are common as women near menopause. A normal menstrual cycle lasts 28 days, plus or minus seven days. Menstrual bleeding is considered irregular if it occurs more frequently than every 21 days or lasts longer than 8 days. Missed, early, or late periods are also considered signs of an irregular cycle. 1 ‘Day 1’ of a cycle is the first day of bleeding, the first day of a period. 2  This bleeding is called menstruation. 3  On average, when periods have become regular, this bleeding lasts for about 5 days. 1 Once you get your period, it may take quite a long time for your body to settle into a regular pattern – maybe up to a year – but most women eventually do get into a regular cycle. 2  It is a good idea to keep track of when your period is due, maybe on a calendar or in your diary. If you have sex close to the end of your period, you could become pregnant since sperm can live for 72 hours in your reproductive tract. There's also the infamous late-in-life pregnancy that can occur during perimenopause, when periods are erratic. Some women's long periods overlap with the beginning of ovulation, which means that you're fertile even though you're menstruating. Say you have a short cycle (21 days, for example) and your period lasts a week. 1 On average, a cycle lasts about 28 days, but it’s quite normal to have a shorter or longer cycle. 2  So your cycle will probably be normal for you even if it is 21 days long, or 35 days! ‘ 3 Day 1’ of a cycle is the first day of bleeding, the first day of a period. 4  This bleeding is called menstruation. 1 Once you get your period, it may take quite a long time for your body to settle into a regular pattern – maybe up to a year – but most women eventually do get into a regular cycle. 2  It is a good idea to keep track of when your period is due, maybe on a calendar or in your diary. It also prepares your body for pregnancy each month. A cycle is counted from the first day of 1 period to the first day of the next period. The average menstrual cycle is 28 days long. Cycles can range anywhere from 21 to 35 days in adults and from 21 to 45 days in young teens. The rise and fall of levels of hormones during the month control the menstrual cycle
http://www.acog.org/Patients/FAQs/Your-First-Period-Especially-for-Teens http://teens.webmd.com/girls/all-about-menstruation?page=2 http://www.acog.org/Patients/FAQs/Your-First-Period-Especially-for-Teens http://www.womenshealth.gov/publications/our-publications/fact-sheet/menstruation.html http://www.everydayhealth.com/pms/irregular-periods.aspx http://www.cyh.com/HealthTopics/HealthTopicDetails.aspx?p=243&np=292&id=2216 http://www.cnn.com/2009/HEALTH/06/17/menstrual.problem.fix/index.html http://www.cyh.com/HealthTopics/HealthTopicDetails.aspx?p=243&np=292&id=2216 http://www.womenshealth.gov/publications/our-publications/fact-sheet/menstruation.html
when does a teens period become consistent