The earlier a girl gets her first period, the greater her risk of developing breast cancer later in life. That's because cancer risk rises the longer a woman's breast tissue is exposed to estrogens released during her menstrual cycle, so fewer pregnancies, later age at first pregnancy and later menopause also are risk factors because they give estrogen more time in a woman's system.
Given reports that more girls are hitting puberty earlier, does that mean that more women might develop breast cancer, and at an earlier age?
The answers, as with many medical questions, are yes, no and maybe.
Girls who begin their periods before age 12 have about a 20 percent higher risk of developing breast cancer compared with those who began their periods after age 14, according to a 1996 paper in the Annual Review of Public Health. Each year that first menstruation is delayed, the risk of breast cancer decreases 9 percent for pre-menopausal women and 4 percent for post-menopausal women, a 2002 French analysis of pooled studies found.
Note the stronger association of early menstruation with pre-menopausal breast cancer than with post-menopausal breast cancer, said Dr. Frank Biro, director of the Division of Adolescent Medicine at Cincinnati Children's Hospital and epidemiology subcommittee chair at Breast Cancer and Environmental Research Centers.
With more girls hitting puberty earlier, he said, "there could be greater rates of pre-menopausal breast cancer going forward." That doesn't mean younger and younger women will be getting breast cancer — breast cancer in your 20s is extremely rare and usually a result of a gene mutation, not early puberty — but rather that breast cancer might become more common among pre-menopausal women.
Diagnoses of breast cancer, after rising in the 1980s and early '90s due to an increase in mammogram screenings, has been stable or floating downward over the past decade. For women older than 60, breast cancer incidence has declined 2.5 percent per year since 1999, perhaps because of a drop in use of hormone replacement therapy among menopausal women, according to the American Cancer Society. For women younger than 60, breast cancer rates have been stable since 1986.
Breast cancer remains a disease that overwhelmingly afflicts older women. Ninety-five percent of new cases and 97 percent of deaths are in women older than 40, the American Cancer Society says. But some researchers say that a trend toward earlier puberty could lead to a bump in breast cancer in the younger set.
These are complicated connections to make. A host of other factors contribute to elevated breast cancer risk, including obesity, lifestyle and genetics, that might also be tied to the age a girl gets her first period. Teasing apart the risk factors will take a long time.
Even whether girls are hitting puberty younger than they used to — and why, and whether it's a bad thing — is up for debate.
The age of first menstruation actually hasn't changed much over the years. According to a 2003 study published in Pediatrics, the median age of menarche is 12.43, not significantly different from the median age in 1973, which was 12.77.
However, it's notable that African-American women, who tend to go through puberty earlier than white women, have higher rates of breast cancer than white women before age 45, but lower rates after 45, Biro said. While one doesn't necessarily cause the other, it's an association to watch, Biro said.