Research has shown than, on average, breast cancers can be found on mammograms about 1.7 years before being detectable on clinical exam.
The guidelines to start annual breast cancer screening at age 40 are agreed upon by the American College of Radiology, the American Cancer Society, the American College of Obstetrics and Gynecology, and the American College of Surgeons.
Every woman should start annual screening mammography at age 40.
If she has a mother or sister who had breast cancer diagnosed before age 50, she should start sooner, usually ten years younger than the age at which her relative was diagnosed, but not before the age of 25.
Breast cancers in women under 50 are usually more aggressive, making early diagnosis crucial.
The recommendation to begin mammography at age 50 came from the United States Preventative Services Task Force.
This has been widely discussed, and most breast cancer experts believe the task force conclusions were based on a flawed data analysis.