Mammography is a type of x-ray imaging which is used to create images of the breast. It uses low dose x-rays; high contrast, high resolution film; and a special x-ray machine specifically designed for taking images of breasts. (Now don't get excited: these are very unerotic x-ray images of squashed breasts.) Most mammography is performed for early detection of breast cancer. It is thought to detect up to 85% of breast cancers in women over 50 years of age (age is one of the risk factors for breast cancer), and can allow discovery of a lump up to two years before a woman or her doctor can feel it. All this is very important because early detection of breast cancer is a key to surviving this horrible disease, which strikes something like one in ten women.
There are two reasons to perform mammography: for screening and for diagnosis. Screening mammography is performed on women who have no symptoms of breast cancer; doctors usually recommend a mammogram every one or two years for women over 40 or, in some countries, 50. (Younger women are at lower risk of developing breast cancer, and they have denser breast tissue, which makes mammograms less effective.) Diagnostic mammography is performed to investigate some kind of abnormality - breast lump or nipple discharge, for example - and to determine the exact size and location of the abnormality and get a view of the surrounding tissue.
Mammography is performed by a technologist, who will position the client so that her breast lies on a surface below which is a film; the technologist then compresses the breast with a paddle so that it's flattened, reducing tissue overlap and thinning the tissue to allow the use of a lower x-ray dose. Mammograms I've had - all done for screening purposes - generally involve squashing each breast twice, once from the top and once from a 45 degree angle. The compression is rather painful, and the way the x-ray technician scoots behind a protective barrier is always frightening, but I have read claims that radiation doses from mammography are similar to those of dental x-rays, and that it has been estimated that if 10,000 women have yearly mammograms for ten years, the radiation from mammograms will cause one additional breast cancer death. More compelling evidence for me is the fact that one course of radiation therapy for breast cancer - usually the first line of defense for cancers that cannot be completely removed surgically - exposes a woman to levels of radiation several hundred times higher than what she would get from having annual screening mammography for 50 years (from age 40 to 90).
I'd be remiss if I didn't mention that the value of screening programs in general, and mammography in particular, is currently being debated, and one Danish study recently showed that regular mammograms did not save any lives. But these studies are controversial, and not everyone is convinced. At this point doctors are still recommending regular mammograms. Until that changes, I'd say that ultimately the choice of whether to follow your doctor's advice and have yearly mammograms is a personal one. But you should make that choice based on an informed risk-benefit analysis, not an irrational fear. I have mammograms because I watched my mother die of breast cancer, and I know that I would rather live with this momentary annual indignity and slight risk increase than suffer a long lingering illness and death.
The other thing I need to say is that performing regular breast self-exams is part of breast cancer early detection. If, like me, you don't like to do them yourself - each slight lump sends me into a tailspin of fear - go see your doctor every six months and have her do them. And remember that lifestyle choices, like smoking, lack of exercise, diet high in saturated fats, and excess alcohol intake all increase your risk of developing breast or other cancers. Okay, I'm done lecturing now.
If you've never had a mammogram done before, you might be nervous. Good informative websites include