A mammogram is a low-dose x-ray exam of the breasts to look for changes that are not normal. The results are recorded on x-ray film or directly into a computer for a doctor called a radiologist to examine.

You stand in front of a special x-ray machine. The person who takes the x-rays, called a radiologic technician, places your breasts, one at a time, between an x-ray plate and a plastic plate. These plates are attached to the x-ray machine and compress the breasts to flatten them. This spreads the breast tissue out to obtain a clearer picture. You will feel pressure on your breast for a few seconds. It may cause you some discomfort; you might feel squeezed or pinched. This feeling only lasts for a few seconds, and the flatter your breast, the better the picture. Most often, two pictures are taken of each breast — one from the side and one from above. A screening mammogram takes about 20 minutes from start to finish.

http://womenshealth.gov/publications/our-publications/fact-sheet/mammograms.cfm