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updated 4/17/2007 6:58:56 PM ET 2007-04-17T22:58:56

Q: My daughter is 12 years old; many of her friends have already started their periods. Is she late and should I worry?

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A: I certainly don't think there's any need for concern; the median age for a girl's first period in the U.S. is 12.43 years. This may be surprising, since the media and seventh grade parents are spreading the word that the age of the onset of periods has gone down (causing a general lament …“what is happening to our sweet little girls”).

The National Diet and Nutrition Survey found no significant change in the median age of the onset of periods (menarche) in the USA over the past 30 years, except among the non-Hispanic black population, in whom menarche occurs 5.5 months earlier than it did 30 years ago. In general, only ten percent of girls are menstruating by the age of 11.11 years and 90 percent are menstruating by age 13.75 years.

I tell moms, and their daughters that they can roughly figure out when the first period will "arrive" based on when their breast buds develop (thelarche). A girl's first period usually occurs within two to three years after that early breast roundness appears. However, we know that the interval can be longer (three years or more) in girls who have early onset of breast budding (before the age of 8) compared to girls with a later onset of breast budding.

There's also a misconception among parents, young girls, and doctors that it's normal for menstrual cycles to be very irregular throughout adolescence. Although early menstrual cycles often occur without an ovulation that “allows” for pregnancy (thank goodness), most girls bleed every 21 to 45 days, even in the first year after they begin their periods. The 45 day cycles are more likely to occur in the early years.  But by the third year after menarche, 60 to 80 percent of menstrual cycles are 21 to 34 days long (which is the same as what we'd expect in adults). A young woman's “usual and customary” cycle length, that will continue through the next decades of her reproductive life, should be established six years after the onset of her menarche (i.e. at 19 to 20 years of age).

You should not be concerned about your daughter's cycles unless:

  • She hasn't started her period within three years of developing breasts.
  • She hasn't started her period by the age of 13 and shows no signs of breast budding or pubic hair development (pubertal sexual changes).
  • She hasn't started her period by the age of 14 and also has signs of excess hair growth.
  • She hasn't started her period by the age of 14 and exercises excessively or has an eating disorder.
  • She hasn't started her period by the age of 14, but has monthly cramps. This could be a sign of an imperforatehymen, which is preventing the visible flow of menstrual blood.
  • She hasn't started her period by the age of 15.
  • After the initial period her cycles are very irregular.
  • Her menstrual cycles have started but then don't occur for 90 days.

If your daughter has any of the above, make sure she sees a physician (usually a gynecologist). There can be endocrine and medical conditions causing lack of and/ or irregular periods.

Finally, I’m often asked (on the subject of early periods in adolescent girls) “what is the normal amount of bleeding?” Most young girls don't know how to assess this, any blood they see on a pad or underwear, especially in the beginning, seems humongous. The best way to ascertain if your daughter is bleeding too much is to find out how often she changes her pad or tampon. Her period is considered excessive if she changes these sanitary products more than once every one to two hours, especially if her flow lasts more than seven days. Make sure you notify her doctor, as it could be a symptom of a bleeding disorder called von Willebrand’s Disease, which is present in one percent of the population.

Dr. Reichman’s Bottom Line: Most girls get their period by the age of 13 but there's usually no need for concern until 15 (as long as pubertal development seems normal). Irregularity and lack of periods in the mid to late teens should be considered an important “vital sign” and signals the need to seek medical expertise.

Dr. Judith Reichman, the TODAY show's medical contributor on women's health, has practiced obstetrics and gynecology for more than 20 years. You will find many answers to your questions in her latest book, "Slow Your Clock Down: The Complete Guide to a Healthy, Younger You," which is now available in paperback. It is published by William Morrow, a division of HarperCollins.

PLEASE NOTE: The information in this column should not be construed as providing specific medical advice, but rather to offer readers information to better understand their lives and health. It is not intended to provide an alternative to professional treatment or to replace the services of a physician.

© 2013 MSNBC Interactive.  Reprints

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