Initial research suggests that obese and overweight women are at increased risk of getting suspicious results from screening mammograms. On the other hand, mammograms don't miss any more cancers in heavy women than in thin women.
The problem is that the more a woman weighs, the greater the risk that her mammogram will have false positive results. This reduction in accuracy translates into more tests and more anxiety for the women.
At the personal level, it's not a huge increase in risk. Overall, a woman faces about a 1 in 10 chance of getting a false positive result on a screening mammogram. Obesity increases that risk from 10% to 12%.