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Dr. Jones: "Help! My daughter is 10 and she's developing breasts. Is this too soon? I'm not ready for this." This is Dr. Kirtly Jones from obstetrics and gynecology at Â鶹ѧÉú¾«Æ·°æ Care and this is puberty on The Scope.

Announcer: Covering all aspects of women's health, this is "The Seven Domains of Women's Health" with Dr. Kirtly Jones on The Scope.

Dr. Jones: Many years ago, a dear friend of mine called me with a worried question. Her 10-year-old, flat-chested daughter had developed a small, tender bump under one nipple. "Is it cancer?" she asked. I replied, "Oh, no. It's something much more frightening. It's puberty." But what is puberty? How old are girls when it usually happens? And is it occurring earlier in the US?

Well, puberty is when the sleeping gonads, ovaries, and testicles of boys and girls wake up to begin the coordinated development of sexual tissues. In girls, it begins with breast development called thelarche, followed closely by pubic hair called pubarche. And about two years after the beginning of breast development in the normal pubertal process, the first menstrual period starts, menarche. This waking up of the sleeping gonads is preceded by a year or two by the maturing of the adrenal glands called adrenarche. All of our lovely developing arches, I'm waiting for an arche to name the start of the development of the mature brain.

Anyway, all of these arches, menarche is the easiest to reliably record and remember. Most women, or their mothers, don't know or recognize the first beginning of breast buds. But the first period is more memorable and there are many reasons why a culture may want to record the age of menarche in its girls. Not the least of which is their potential fertility and marriageability.

The average age of menarche in the early 1800s was about 17, as reported in Norwegian records. There was a steady and significant decline in the age of menarche over the past 200 years in European records. Although one researcher in the US noted about 1900 that girls with "greater culture and education" had earlier menarche by about a year, it was probably better nutrition that contributed to the earlier menarche. "Cultured white girls" had earlier menarche than Irish immigrant girls or African-American girls. And that's probably not the culture that made a difference, but the nutrition.

Contributing to earlier menarche is early childhood nutrition and pre-pubertal nutrition. So, is the age of menarche still falling? And the answer is yes, all over the world, but not so quickly. The National Health and Nutrition Examination Survey, NHANES, found that the average age of menarche in the US dropped about one year from girls born in 1920 to women born in 1980. African-American girls had the biggest drop from 13.6 years to 12.2 years. Non-Hispanic white girls had a decline from 13.3 years to 12.5 years. And Hispanic girls were about in the middle. In Japan, Japanese girls had their change of menarche drop from 13.8 in girls born in the '30s to 12.2 for girls born in the '80s.

But, you know, 13, 12 is not such a big deal, but to think of it a different way, the percent of US girls who've had their first period by age 10 was zero for girls born in the '20s and is 2% to 3% for girls born in 1980.

So why is this happening? Is it good nutrition, our fruits, veggies, and protein? Or is it bad nutrition? Well, body weight is a contributing factor to the age of menarche and we know that kids are taller now than they were 80 years ago. The most recent changes in the past 30 years have not been so much taller kids, but fatter kids. The likelihood of reaching menarche in girls now is most likely associated and most highly associated with pre-pubertal adiposity, or fatness. Although, some people even question this.

So sweet drinks in cans and boxes are associated with earlier periods. A few separate studies have debated whether it's the sodas, caffeinated, sugared, or artificially sweetened, and whether they're associated with early menarche. And there are some debates as to whether it's the caffeine, the sugar, or the artificial sweeteners, but one big study suggests that sweet drinks in cans do make puberty earlier. We do know that kids that consume artificially sweetened sodas are more likely to be overweight. But is it the overweight kids are more likely to drink artificially sweet drinks?

Or is it the bisphenol A that lines the soda cans? Well, BPA is a plastic with some estrogen-like activity that's been a concern for all of us. One study, though, looked at BPA levels in the urine of kids and recorded that the menarche, when controlling for race and fatness, didn't seem to be different based on their BPA levels. But there is still a concern about the artificial chemicals in the environment in the lotions and potions we put on our kids that may have some effect on their puberty.

So how does this all matter? Well, first, as the age of menarche goes down, the number of kids who have menarche before 10 goes up. Developing breasts at seven and periods at nine is hard for girls that we still consider as little kids. We also know that earlier exposure to estrogens is a small factor in the risk of breast cancer. When could early puberty be a medical problem? Well, if your daughter develops breasts before the age of seven, you should talk to her pediatrician.

Now, is there anything we can do about it? If pre-pubertal adiposity, fatness, is the number one risk factor for earlier breast development and periods, we should take care with what our kids eat and drink and keep them active. We should have a say about what food is available at their schools, such as sodas and juices and high-calorie snacks in the machines. We should try to decrease screen time and increase outdoor time. And, most importantly, because we cannot control everything, we need to be ready to have the talk and support our girls as girls as they make this transition. And thanks for joining us on The Scope.

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