If you are confused about whether or not to have annual mammograms during your forties, no wonder. Doctors can't agree on this either. Consider the opposing positions:

The reduction in this age group is smaller than that in women 50 years of age or older, is subject to greater uncertainty about the exact reduction in risk, and comes with the risk for potential harms (such as false-positive and false-negative results, exposure to radiation, discomfort, and anxiety).
--Amir Qaseem, M.D., Ph.D., American College of Physicians

This was an incredibly irresponsible decision by the American College of Physicians. They clearly don't understand the screening trials of mammography and they don't understand the data. They just totally misinterpreted things.
--Radiologist Daniel B. Kopans, M.D., of Harvard

The new ACP guidelines suggesting that routine annual mammograms for 40-somethings reopens a debate that was recently 'settled' in favor of yearly exams for through the 40's. Conventional wisdom held that while 40-ish women were less likely to get breast cancer than their older colleagues, their cancers tended to be more aggressive and more frequent screenings were useful to catch faster-growing tumors earlier by looking for them more often.

Women at higher risk for breast cancer due to an inherited predisposition should have earlier screenings PLUS breast MRI scanning. MRI screening, while expensive, is now recommended for all women diagnosed with breast cancer to look for other tumor foci. This recommendation may ultimately extend to women with dense breast tissue on mammogram; breast cancers are harder to visualize in such breasts by mammograms and these women are known to be at a much higher risk for developing malignant tumors, a risk believed to be equivalent to women with BRCA mutations.