Of the nearly 30 million American women who undergo screening mammograms every year, up to 11 percent receive “probably benign” test results — and therefore are asked to come back for a follow-up mammogram in three to six months. But according to a new study by UC Davis researchers, such frequent re-testing may be unnecessary. The study appears in tomorrow’s issue of the Journal of the National Cancer Institute. The UC Davis researchers examined the mammography records of nearly 60,000 women enrolled in the national Women’s Health Initiative project, one of the largest preventive health studies in the United States. Among the women who had probably benign mammograms, only 1 percent went on to develop breast cancer within two years, the investigation found.