The economy has been growing for the past eight years. What will it take to keep climbing?

According to a recent Standard and Poor's study, one key to continued success is this: Hire more women.

"We've seen a slowdown in women entering and being retained in workforce, and this productivity boost we could get from these women would give the U.S. a boost, " S&P Global's U.S. chief economist Beth Ann Bovino told CNBC's "On The Money" in a recent interview. Bovino co-authored an S&P report that analyzed trends in the labor market, and how they impact female workers.

According to S&P's data, the labor force participation rate for women of prime working age in 1990 was 74 percent, but by 2000 that rate had declined to 60 percent. Today, female labor force participation stands at a mere 57 percent.

So where are the women, and why the sharp decline?

"Probably the biggest burden is child care," Bovino told CNBC. While she credits men today with helping more, "women (still) do the lion's share of child care and elder care as well." Maternity and family leave also impacts the decision for women to rejoin the workforce, she added.

Bovino said the S&P study found that 39 percent of mothers took "a lot of time out of the workforce when they have children, and 25% don't come back." Her research found 24 percent of fathers take significant time off for child or family care.

Bovino suggested flex time could help encourage women to go back to work, if they choose to. "Open up the opportunities for flexible work schedules I think that would help businesses and women. Women because they could manage home and work," she said, and for businesses it would reduce overhead costs.

The report's findings said that "a dual-pronged effort of increasing entry and retention of more women to the workforce, particularly those professions traditionally filled by men, represents a substantial stronger opportunity for the world's principal economy."

Bovino stated that "growth potential has been incredibly slow. While the GDP rate has averaged around 2.5 percent during the recovery, our potential growth outlook is down one percent. We expect only 1.8 percent growth."

S&P Global estimated that by adding more women workers to the U.S. economy, it could add "5 to 10 percent to nominal GDP in just a few decades."

In the wake of comprehensive tax reform, Bovino pointed to the fact the new tax law "does offer more child care credit, that is one step in the right direction." Still, the data found a noticeable wage gap that revealed women earn 83 percent of what a comparable male full-time worker makes.

Bovino said another source of progress would be if more girls and women studied science, technology, engineering and math. "The fields that have been considerably male domain, you can still see that," the economist said.

Only 14 percent of women have studied a STEM field, a statistic Bovino mentioned was "half the rate of men." However, with the employment rate for STEM-educated adults is 88 percent, those fields have a great upside for "higher wages and higher employment opportunities," she added.

On the Money airs on CNBC Saturday at 5:30 am ET, or check listings for air times in local markets.