In the last two decades, breast cancer screening with mammography exams has reduced the number of deaths from the disease by one-third. Still, there has recently been confusion over when women should begin screening (age 40? 50?), and some women aren’t undergoing routine testing at all. In sorting fact from fiction, here are five things every woman should know about this very important—and life-saving—diagnostic test.

1. Don’t wait

According to the American Cancer Society, women should begin annual mammography exams at the age of 40. Yearly testing should continue until the age of 70.

2. Go to a specialist

Certain factors, such as taking hormone replacement therapy, breast density, and time since last mammogram, can influence the reliability of mammography test results. “Women should go to facilities with radiologists who have special training in breast imaging,” says Stephen Taplin, a senior scientist at the National Cancer Institute.

3. Beware false-negatives

Breast density can affect test results. The denser breast tissue is, the more difficult it is to pinpoint breast cancer in its early stages. Younger women, as well as those who have a family history, are particularly prone to more dense tissue. Although self-exam is no replacement for a mammogram or clinical exam, knowing your body can help to differentiate when something abnormal is present.

4. Understand false-positives

Some studies have shown that a previous breast biopsy can affect mammography interpretation. Taplin reported in the May issue of the Journal of the National Cancer Institute that biopsy changes breast tissue and can reduce specificity of mammography. “This is somewhat controversial,” says Taplin, “The bottom line is that women with a prior benign biopsy are more likely to have a subsequent false positive mammogram, but it does not appear that sensitivity of the test is lower.”

5. Know the risks

As counterintuitive as it may seem, screening for cancer has a downside. According to the National Cancer Institute, it is not possible to differentiate which breast cancers require treatment and which can exist in the patient without causing harm. Therefore, patients may undergo treatments like chemotherapy and radiation without need. Also, there is the possibility that a mammogram will show the presence of an untreatable form of breast cancer and the patient will be faced with diagnosis of a terminal illness. The pros of screening outweigh the cons but it’s important to know what to expect.