Despite significant gains in the workforce over the past quarter century, women continue to be less prepared to retire than men, according to Hewitt Associates.

In a study released this week, the Illinois-based human resources consulting firm found American women on average need to save two percent more of their annual income over 30 years to achieve the same standard of living as men in retirement.

That's because women tend to make less and live longer, researchers said. An analysis of two million employees at 72 large U.S. companies found women earned an average salary of $57,000 a year, compared to $84,000 for men, the firm reported.

Higher medical costs and less risky, low-return investments also tend to leave women with less savings at the end of their careers.

"Once you get into your 30s and 40s and later, it gets very difficult to start saving enough," Alison Borland, an author of the study, said in a statement. "Women tend to be a little more risk averse, more fearful of losing money."

Most women needed only minor changes to their investing behavior, such as investing earlier in their careers, to save enough to retire comfortably, the study said.