Precancerous growths on the genitals are becoming more common in women, particularly those in their 40s, a medical group said Thursday, reporting a fourfold increase in women over all from 1973 to 2000.

Most of the growths are caused by the human papilloma virus, or HPV, a common sexually transmitted virus also known to cause cancers of the cervix, penis, anus and parts of the throat. The genital precancers do not always turn into cancer, but doctors say they should be removed anyway because it is impossible to predict which ones will progress.

The growths, called vulvar intraepithelial neoplasia, or VIN, form on the external parts of the genital area, or vulva, including the labia and clitoris. The anus and thighs may also be affected. Surgery, lasers or drugs can be used to get rid of the growths.

The reason for the increase in genital precancers is not known, but some doctors think it is that people are having more sexual partners than in the past and more chances to be exposed to HPV. More than half of sexually active men and women become infected, and 80 percent of women have been exposed by age 50, researchers estimate.