Evidence is mounting that many women with early-stage breast cancer are opting for mastectomies instead of having breast-conserving surgeries that offer the same odds of survival.

A new analysis of more than 1.2 million breast cancer patients in the U.S. found that 37.8 percent of women who were eligible for breast-conserving surgery wound up getting mastectomies in 2011, the most recent year for which data were available. That’s up from 34.3 percent in 1998, an increase large enough to be statistically significant, according to a report published Wednesday in JAMA Surgery.

Among the women who had mastectomies during the 14-year study period, 45 percent had total mastectomies, 34.7 percent had modified radical mastectomies and 0.8 percent had radical mastectomies.

But most of the overall increase in mastectomies could be traced to the 19.5 percent who opted for bilateral mastectomies, and doctors are struggling to understand why

Long-term tracking of breast cancer patients has shown that women diagnosed with early-stage tumors fare just as well with the more limited breast-conserving surgery as they do with mastectomies.

Doctors from Memorial Sloan-Kettering Cancer Center and several Harvard-affiliated schools and hospitals developed a survey about the motivations of women between the ages of 26 and 40 who decided to treat their early-stage breast cancers by removing both breasts. The survey revealed that 98 percent of the women felt it was “extremely important” or “very important” to eliminate the risk of developing a future cancer in their healthy breast; 95 percent felt the procedure would give them peace of mind; and 94 percent thought it would improve their odds for a cure, according to a 2013 report in Annals of Internal Medicine.