(USA TODAY) -- Doctors have long promoted IUDs, or intrauterine devices, as an effective way to prevent pregnancy. Now, in a finding that has surprised even the experts, research suggests that IUDs have an unexpected benefit: preventing cervical cancer.

According to research released Monday, women who had used an IUD had almost half the risk of cervical cancer as other women. The international analysis, published in The Lancet Oncology, combined data from 26 studies with a total of more than 20,000 women.

Although doctors have known for some time that women who use IUDs have a lower risk of cancer of the lining of the uterus, researchers say they never suspected that the devices might protect against tumors of the cervix, says Xavier Castellsagu of Institut Catal d'Oncologia in Barcelona, Spain. In fact, researchers believed IUDs might increase cervical cancer risk.

Cervical cancer is caused by infection with the human papillomavirus (HPV), a family of viruses that spread easily through sexual contact.

IUDs are highly effective, preventing more than 99% of pregnancies, says Ranit Mishori, an assistant professor of family medicine at the Georgetown University School of Medicine.

The study's authors say they can't fully explain why an IUD might fight cancer.

Unlike a condom, the IUD doesn't create a physical barrier against the virus. In the study, using an IUD didn't affect a woman's risk of being infected with HPV, Castellsagu says. But it's possible that an IUD prevents the virus from progressing to cancer, experts say.

By causing a chronic, low-level irritation in the cervix, an IUD may rev up a woman's immune system, as if her body were trying to heal a wound, according to an accompanying editorial by Karl Ulrich Petry, a researcher at the Klinicum Wolfsburg in Germany. That boost may be enough to clear persistent HPV infections and even get rid of precancerous lesions.

Still, it's too soon to begin recommending IUDs for cervical cancer prevention, says Carol Brown, a cervical cancer specialist at New York's Memorial Sloan-Kettering Cancer Center, who wasn't involved in the new study. Researchers will need to confirm this finding with larger, more rigorous trials before they can be sure about the IUD's benefits.

Brown says women already can reliably protect themselves from cervical cancer through regular screenings.

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