WASHINGTON — The Food and Drug Administration took the rare step Thursday of urging doctors to stop performing a surgical procedure used on tens of thousands of woman each year to remove uterine growths, saying the practice risks spreading hidden cancers within a woman's body.The procedure, know as power morcellation, has long been used in laparoscopic operations to remove fibroid tumors from the uterus, or to remove the uterus itself. It involves inserting an electric device into the abdomen and slicing up tissue to remove it through a small incision. The surgery is far less invasive than traditional abdominal operations.But the FDA on Thursday agreed with a growing chorus of researchers and clinicians who oppose the procedure, arguing it can recklessly spread undetected cancers throughout the body and make the disease more lethal in the process. The agency isn't seeking to ban the practice or the roughly two dozen FDA-approved devices used to perform it, but hospitals and gynecologists are likely to abandon the procedure anyway, because of potential liabilities.The FDA said its analysis determined that an estimated 1 in 350 women who undergo morcellation have an unsuspected form of uterine cancer called uterine sarcoma."The existence of the risk is not new," said William Maisel, chief scientist at the FDA's Center for Devices and Radiological Health. "What is new is that the magnitude of the risk appears to be higher than was appreciated by the clinical community."