Fat cells make estrogen. And when breast cells are exposed to extra estrogen over time, the risk of developing breast cancer is higher. One 2006 study found that women who gained about 55 pounds or more after the age of 18 had a 45 percent increased risk of developing breast cancer compared with those who maintained their weight. Gaining weight after menopause can also increase a woman’s risk. That same study found that putting on 22 pounds after menopause increased the risk of developing breast cancer by 18 percent.