A new study looks at more than 400 pairs of twins in Europe who had breast cancer before age 50.
If a girl began developing breasts early, and if she was tall and slim for her age at 10, she was at increased risk, reports study researcher Thomas Mack, MD, professor of preventive medicine at the University of Southern California Keck School of Medicine in Los Angeles.
This pattern was true even when there was no family history of breast cancer.
"We've known that age of menarche [a girl's first period] is a risk factor for breast cancer because it is linked with her body's lifetime exposure to estrogen," Mack tells WebMD. Breast development typically starts before menarche.
"It could be that if her breasts are not fully developed, they are more susceptible to hormones that come at her first period," he says.
A girl's height before puberty may be due to high levels of a hormone known as insulin-like growth factor (IGF)-I in the blood. High levels of IGF-I have also been linked with increased risk of premenopausal breast cancer.
In their study, Mack and colleagues tracked down 400 women who had breast cancer before age 50 -- and their twin sisters who did not have breast cancer at that time. They interviewed the women and asked them to complete questionnaires about their personal health history, especially their prepuberty years.
They found that a woman's risk of breast cancer was increased by 44% if she was less heavy than her twin sister and by 27% if she was the taller of the two. In fact, her risk increased by 53% if she had developed breasts earlier and by 79% if she had a smaller waist-to-hip ratio at age 20.
However, obese women are known to be at higher risk of postmenopausal cancer. "There's all kind of speculation why, but we don't really know," says Mack.
The findings regarding height -- and the possible link with IGF-I -- are especially intriguing to LaMar McGinnis, MD, senior medical consultant with the American Cancer Society. He reviewed the study for WebMD.
"Insulin growth factor is an inherited factor," McGinnis tells WebMD. "This may be something to look for. There may be something there."