U.S. researchers have found the risk of infection with the human papilloma virus (HPV) -- a virus associated with cervical cancer -- increases ten-fold for each new sexual relationship a woman has per month.

The scientists from the University of California, San Francisco, have been monitoring more than 800 women who attended family planning clinics in 1990.

Their findings suggest that more than half of sexually active young women in the U.S. will become infected with HPV over a three-year period. The infection figures are thought to be similar in Europe, BBC News reported.

Other findings of the research:

* Women who took oral contraceptives cut their relative risk of HPV infection in half.

* Infection with herpes simplex virus also increased the risk, as did a history of vulvar warts.

* Up to 90% of young women clear the HPV virus from their systems within 36 months. However, many become reinfected.

However, the same study has contradicted previous research which suggests that infection with the virus is certain to trigger cellular changes that can lead to cancer.

The researchers found that, over a five-year period, only approximately 30% of women infected with HVP showed signs of changes to the cells of the cervix. These changes, called LSIL (low-grade intra-epithelial lesions) are benign in themselves, but can be the first step to the development of cervical cancer.

It appears that women are more likely to develop LSIL in the first year after HPV infection. The risk tails off after four years. Daily cigarette smoking appears to increase the risk.

Experts say the study, which was published in this month’s issue of the Journal of the American Medical Association (JAMA), reinforces the importance of safe sex and use of condoms.