The onset of menstrual cycles, or menarche, is a defining moment in a girl’s physical development. It is important to keep tabs on this important health marker throughout adolescence and beyond. It is so significant, in fact, that the American Academy of Pediatrics urges healthcare workers to treat the menstrual cycle as another vital sign, along with pulse, temperature, respiration and blood pressure.
That said, many teens and their mothers are not sure what a “normal” menstrual pattern is since they are only familiar with their own cycles. In the United States, most girls start menstruating between ages 12 and 13. A girl can expect to have her first period two to three years after the onset of breast development. If she has not started menstruating by age 15 or within three years of the start of breast development, she should be evaluated by her doctor. 
The first couple of years of menstruation are usually irregular since initially most girls are not reliably ovulating, but how irregular is irregular? Even in those early years, most cycles range from 21 to 45 days from the start of one period to the start of the next. In general, “early bloomers” take less time to settleinto regular cycles than late bloomers. By six years after the onset of menstruation, most young women have established a predictable cycle. 
If a teenager is going more than three months between cycles, even early on, it is worth a visit to the doctor. Her pattern may still be normal, but her physician will want to check for obesity, hormone imbalances, and possibly even pregnancy, all of which can wreak havoc on menstruation. Athletic girls can also suffer from a condition known as the female athlete triad, characterized by inadequate calories, decreased menstruation and low bone density, which can lead to fractures. 
On the other end of the spectrum, excessive menstrual bleeding can be dangerous. A girl who needs to change a pad or tampon more often than every hour or two, has clots like golf balls or has significant flow for more than seven days at a time should be evaluated by a physician. Her doctor will likely look for an underlying cause and make sure she is not becoming anemic from the loss of blood.
Encourage your daughter to chart her cycles on her phone or calendar and to bring any concerns to your attention. Her pediatrician can help manage this important aspect of her health.
Dr. Suzanne Condron is a board-certified pediatrician at Kelsey-Seybold Clinic – Fort Bend Medical and Diagnostic Center whose clinical interests include obesity, nutrition, allergies, asthma, childhood development, literacy, infectious diseases and preventive medicine.
 American Academy of Pediatrics, Committee on Adolescence, American College of Obstetricians and Gynecologists, Committee on Adolescent Health Care. Menstruation in girls and adolescents: Using the menstrual cycle as a vital sign. Pediatrics. Nov 2006,118 (5) 2245-2250.
 Golden, NH, Abrams, SA, AAP Committee on Nutrition. Optimizing bone health in children and adolescents. Pediatrics. Oct 2014, 134 ( 4) e 1229-e1243.