(Reuters) — UnitedHealth Group (NYSE:UNH), the largest health insurer in the U.S., is placing tighter controls on its coverage of hysterectomies after a device called a morcellator was linked to the spread of undiagnosed cancer cells.

In an update to its reimbursement guidelines, UnitedHealth said that, starting April 6, it will require physicians to obtain authorization before carrying out certain types of hysterectomies, a procedure in which a woman’s uterus is removed for reasons ranging from fibroids or endometriosis to chronic pelvic pain.

UnitedHealth said it will not require prior authorization for vaginal hysterectomies, in which the uterus is removed through the vagina, when done on an outpatient basis.

In its update, the insurer cites the American Congress of Obstetricians & Gynecologists as identifying vaginal hysterectomies as the preferred method.

The FDA issued an advisory last April recommending that surgeons avoid the use of laparoscopic power morcellators in performing hysterectomies because of evidence the technique could spread cancerous tissue. Morcellators are devices used in laparoscopic surgery to cut up tissue so that it can be removed through small incision sites.

The FDA said a vaginal hysterectomy usually yields comparable or better results with fewer complications than laparoscopic or abdominal hysterectomies.

Late last year the federal safety watchdog added a “black box” warning to morcellators, advising doctors that the devices be avoided in nearly all fibroid-removal procedures.