Researchers examined mammogram results among more than 16,000 women. They found that those taking hormones were more likely to have an abnormal result on their mammogram. That meant additional follow up testing was needed.
In addition, 10 percent of hormone users underwent a breast biopsy to test for possible cancer, compared to just six percent of those taking the placebo.
Doctors say the hormones increase breast density which can make mammograms difficult to interpret.