Uterine fibroids overview Fibroids are tumors made of smooth muscle cells and fibrous connective tissue. They develop in the uterus. It is estimated that 70 to 80 percent of women will develop fibroids in their lifetime — however, not everyone will develop symptoms or require treatment. Image of Uterine Fibroids The most important characteristic of fibroids is that they’re almost always benign, or noncancerous. That said, some fibroids begin as cancer — but benign fibroids can’t become cancer.