Rates of breast cancer in women under 50 have increased more than 15% in the last two decades, a rise that is almost entirely driven by an increase in the most common type, estrogen receptor-positive breast cancer. Numerous factors may be to blame — including obesity, alcohol consumption, genetics and some hormonal birth control — but some doctors suspect the trend is related to more people in the U.S. getting their periods earlier and having their first baby later. |