Women who went through at least five years of fertility issues before becoming pregnant have an increased risk of heart disease, researchers say.

The women were considered “subfertile” if they are unable to get pregnant for over a year. Previous research has indicated that subfertility may be caused by thyroid disease, irregular menstrual cycles and obesity. Polycystic ovarian syndrome may also play a role.

“We need to find out what it is about being subfertile that puts a woman at excess risk of heart disease,” said study co-author Dr. Nisha Parikh, a cardiologist from the University of Hawaii.

Nearly 863,000 Swedish women were followed from 1983 to 2005. Over 3,000 of the study participants developed heart disease over the course of the study. Researchers found five of every 10,000 subfertile women developed heart disease, compared to three of every 10,000 women with no fertility problems.

The findings were published in the journal Human Reproduction.

Over 7 million women of childbearing age experience fertility problems, according to the Centers for Disease Control and Prevention.