The U.S. Food and Drug Administration (FDA) is warning doctors not to use morcellators to treat uterine fibroids because the device might actually spread cancer in women, NBC News reports.

Morcellators are tools used to grind up fibroids in a woman’s uterus so they can be easily removed. The problem is that there is no good test that can differentiate between fibroids—usually benign, harmless growths—or a pre-cancerous tumor.

Doctors say not knowing the nature of these growths, and then breaking them up into chunks, could potentially spread undiagnosed cancer through a woman’s body.

“We believe that in the vast majority of women, the procedure should not be performed,” said William Maisel, MD, PhD, from the FDA’s Center for Devices and Radiological Health.

But the morcellator is still considered safe to use on a small number of women with fibroids, including younger women who want to maintain their ability to have children, or women who have been living with their fibroids for a long time and it’s clear to doctors that the tumors aren’t cancerous.

Did you know that African-American women are far more likely to have uterine fibroids than women of other races? Click here to learn why.