Fibroids and their location in the uterus

The uterus is built mostly of smooth muscle, the kind of muscle that works on its own without you telling it to, and of connective tissue, the scaffolding that holds everything in place. When a group of those fibers organizes itself into a firm, well-defined lump, that's a uterine fibroid, also called a leiomyoma. It's a benign tumor: it takes up space and it grows, but it doesn't invade or behave like a cancer.

Think of the uterine wall the way you'd think of a thick wall in a house. A fibroid can stay buried inside that wall, it can push inward and bulge into the hollow space at the center, the uterine cavity, or it can push outward and raise a bump on the outer surface. Sometimes there's just one, sometimes several; some are tiny and others get very large. Plenty of women live with them and never find out; in others they come with heavy bleeding, a feeling of pressure, or pain.