From Boston University:

Here’s a shocking fact: nearly 80 percent of all women will develop uterine fibroids at some point in their lifetime, making them the most common type of benign tumor in the reproductive system—and yet scientists still don’t know what causes them. Many people have fibroids without ever knowing it, but others experience symptoms like heavy menstrual bleeding and cramping. Scientists know that they form in the smooth muscle cells of the uterus, but that’s about it. And it’s not the only reproductive health issue with glaring gaps in our knowledge.