One of the longest-lasting and most effective forms of female contraception may be offering an unexpected health benefit to the women who use it.

A new analysis of intrauterine devices (IUDs) has found women using the contraceptive were significantly less likely to develop cervical cancer, with IUDs reducing incidences of the cancer by approximately one-third.

"The pattern we found was stunning. It was not subtle at all," says preventive medicine specialist Victoria Cortessis from the University of Southern California.

"The possibility that a woman could experience some help with cancer control at the same time she is making contraception decisions could potentially be very, very impactful."

Cortessis and fellow researchers reviewed data from 16 observational studies monitoring more than 12,000 women, identifying both participants' use of IUDs and their incidence of cervical cancer, which is the fourth-most common cancer in women worldwide.

What they found was that among women who took part in the studies, those using IUDs were 36 percent less likely to get cervical cancer than women who didn't use the contraceptive.

Of course, meta-analyses like this are only observational in nature – neither the new research nor the studies it draws from are demonstrating any kind of causative effect.

But, nonetheless, it's a striking, unexpected result that the researchers say definitely warrants further investigation.

"It looks real. It smells real," Cortessis told Live Science.

"[B]ut to be really convinced, we need to go back and do studies to find a mechanism."

What that mechanism is exactly, nobody's sure, but the team speculates the placement of an IUD may somehow stimulate an immune response in the cervix, leading the body to protect itself against any existing human papillomavirus (HPV) infection – the virus that causes more than 70 percent of all cervical cancer cases.

"[Previous] data say the presence of the IUD in the uterus stimulates an immune response, and that immune response very, very substantially destroys sperm and keeps sperm from reaching the egg," Cortessis explained to HealthDay.

"It stands to reason the IUD might influence other immune phenomenon."

Another hypothesis is that when IUDs are removed from the body, a scraping effect could take out infected cells at the same time, which could potentially help lower the risk of cancerous tissue developing.

Regardless of what makes the reduced cancer risk happen, the sheer size of this gap seen in the data means it's something health researchers will want to look into.

"I would be shocked if it's not a real phenomenon," Cortessis told Time.

"We need to figure out what's going on mechanistically and do some fine tuning and see what kind of use could prevent cervical cancer and integrate that with contraceptive counselling."

The researchers are keen to emphasise that their findings shouldn't be taken as a recommendation that women should use IUDs to lower their chances of getting cervical cancer.

The best way to do that is to have regular cervical screenings and be vaccinated against HPV.

"Screening is everything," Cortessis told Newsweek.

"If a woman has one lifetime screening visit in her entire life, her risk is much lower."

The findings are reported in Obstetrics & Gynecology.