From the article:
In the Women’s Health Initiative study, 10,739 postmenopausal women with hysterectomy were assigned to either conjugated equine estrogen or a placebo. Previous analyses did not show an increase in breast cancer incidence in the women who took estrogen alone after a median follow-up of seven years.
To determine whether the hormone increases the risk of benign proliferative breast disease [condition that is associated with increased risk of breast cancer], Tom Rohan, M.D., Ph.D., of the Albert Einstein College of Medicine in New York and colleagues identified and examined non-cancerous breast biopsies in each of the Women’s Health Initiative trial arms.
A total of 232 cases of benign proliferative breast disease were identified, with 155 cases among the women who took estrogen supplements and 77 in the placebo group. The risk of developing benign disease increased by more than two-fold for women taking conjugated equine estrogen, compared with those taking a placebo.
[…]
Although the women taking conjugated equine estrogen have not yet shown a significant increased risk of breast cancer in the Women’s Health Initiative study, if this hypothesis holds true, they might show increased risk later. Ongoing follow up of the study participants may help to resolve this issue.