Poor women are five times as likely as affluent women to have an unintended birth, new research from the Brookings Institution shows – and that drives inequality.

The difference, however, boils down to contraceptive use, not sexual activity. There is no “sex gap” by income, researchers emphasized. Promiscuity doesn’t vary along class lines.

Access to the most reliable forms of birth control, though, does.

The Brookings study examined fertility outcomes of 3,885 single women, none of whom were trying to get pregnant. Those with incomes below the poverty line were twice as likely to have sex without protection as those with incomes four times the poverty line, data from the National Survey of Family Growth showed.

The failure rate for progestin-releasing IUDs, for example, which can cost up to $1,000, is 0.08 percent, according to the Centers for Disease Control and Prevention. The failure rate for condoms: 12 percent.

“In a sense, inequality starts before birth,” said co-author Richard Reeves, policy director of the Center on Children and Families. “An important part of the policy story is helping parents have children when they’re ready. The life chances of those children will be better as a result.”

A growing body of evidence indicates that limited access to sex education and contraceptives in poor communities widens the income-fertility gap.