Women who have three to six alcoholic drinks a week are at slightly increased risk of developing breast cancer compared with women who don’t drink at all, a new study has found.

The study followed more than 100,000 nurses for up to almost 30 years, according to USA Today. Most previous studies on alcohol and breast cancer have found no increased risk for the disease among women who are light drinkers, the article notes.

The women in the new study who had three to six drinks a week had a 15 percent increased risk of developing breast cancer compared with nondrinkers. That translates into about four additional cases of breast cancer per 1,000 women. A woman’s risk of the disease increased the more she drank, regardless of whether she drank wine, beer or liquor.

In a news release, the study authors said that while the exact way in which alcohol may lead to breast cancer is not known, one probable explanation may involve alcohol’s effect on circulating estrogen levels.

The study appears in the Journal of the American Medical Association.