The nation's largest insurer is putting sharp limits on the most common form of hysterectomies, saying the procedure poses a health threat and that a different form of the surgery has been found to be safer.

UnitedHealthcare (UNH) has sent out a bulletin to physicians saying that it will start denying coverage for laparoscopic or abdominal hysterectomies in April unless the doctor gets pre-authorization.

The insurer says it believes vaginal hysterectomies, which account for only 22% of the more than 500,000 hysterectomies a year, are the preferred form and will not require pre-authorization.

The insurer cited the finding of the American Congress of Obstetricians and Gynecologists that "evidence demonstrates that, in general, vaginal hysterectomy is associated with better outcomes and fewer complications than laparoscopic or abdominal hysterectomies."

Related: Thanks, Obamacare - Health insurers stocks soar

The Food and Drug Administration has also warned against the laparoscopic or abdominal hysterectomies.

The FDA said the device used in those procedures, the laparoscopic power morcellators, risks "spreading unsuspected cancer" even in women who are having non-cancerous fibroid tissue removed, let alone those who are having the procedure in an effort to remove the cancer.

The FDA and the College of Obstetricians and Gynecologists acted after a story and an e-book by the Wall Street Journal, which called public attention the risks posed by the morcellators.

United Healthcare's notice said physicians who do not get preauthorization for the procedure will have their claim for compensation denied. It said the doctors will not be allowed to bill their patients for the procedure.

That will essentially stop physicians from performing the procedure for most of the more than 40 million female members covered by the insurer, since their inability to bill those patients will mean they can't even pay out of pocket for the procedure.