First, the good news. About 58% of today's professional school graduates are female. So yes, women have come a long way in the classroom. But take a look beyond the classroom and too often we're back in the dark ages.

Women still represent only 8% of the top earners at Fortune 500 companies and the

U.S. Bureau of Labor Statistics points to a 6% drop in the number of married women with children under 12 months old in the labor force. Talented women are dropping out of the workforce at alarming rates before they can make partner or board member or CEO.

It's not the individual women who are to blame, says Betty Spence, president of the

National Association for Female Executives. Women, after all, own about 10.4 million of the nation's small-business firms and generate about $1.9 trillion in annual sales, according to the

Center For Women's Business Research.

So if women are more than capable on their own, what's the problem in the corporate world?

Sylvia Ann Hewlett, author of

Off-Ramps and On-Ramps

, is one of the first to map out this mystery.

Some Things Haven't Changed

Hewlett's on-track academic career led her to a fantastic job as an assistant professor at Barnard College. Then her first child shook things up a bit. The college gave her two and a half weeks maternity leave, which wasn't much, but she coped.

Two years later, when Hewlett lost twins in her seventh month of pregnancy, she was given a mere three days off to cope. "Having children

was seen as some kind of expensive private hobby," she says. She took an unpaid leave of absence and got fired. "If someone with a lot of credentials got derailed by this, surely hundreds of thousands of other women are not coping either," she realized.

Today's predominant business model still caters largely to men and is littered with damaging myths like "women aren't risk-takers," and "company loyalty will suffer once the first child is born," says Spence. A common misconception, for example, is that when women "off-ramp," or leave the work force, they leave for about 15 years. In reality, women are eager to get back to their careers after about 2.3 years, says Hewlett.

These misconceptions, combined with an oft-immovable business structure, prevent companies from addressing the unique challenges women face and often push women out of the workforce.

Women are expected to stay in lock-step employment, says Hewlett, which can be almost impossible when family comes into play. The reason so many women aren't making those top positions is that familial demands tend to arrive just as the woman enters the steepest gradient of her career path. "Talk to any person in the Fortune 500 and they will tell you that it's that mid-30s push where you catch that wave or you don't that shapes your career," she points out.

Companies can't afford to lose women at this pivotal time, says Hewlett, but the casualties are still mounting. Two-thirds of highly qualified women either leave the workforce or languish on the sidelines during this crucial period, she says.

"Women have fallen off the radar in the concept of diversity," says Spence. "

Corporate America is not focusing on where the talent and money really is right now: American women."

Taking the On-Ramp

This matters because two generations of baby busts have finally created a hole in the workforce that employers can't afford to leave empty, says Hewlett. Every time a company loses a skilled professional, it costs two times their annual salary to replace them, she says. By 2010, women are predicted to be controlling an estimated $21.3 trillion in the U.S., according to the diversity director at

AXA Equitable, so smart companies are now working to stay ahead of the curve by finding ways to retain the lucrative female demographic.

Top companies have recently introduced a slew of programs to create a more sustainable career model for women.

Deloitte & Touche

, for example, was "hemorrhaging female employees," says Spence, so they began offering paid leave and welcomed women back after longer absences. Companies like

IBM

(IBM) - Get Report

will reserve a female employee's job for a few years. These modified business models are winning the loyalty of women, says Spence, and companies are seeing profits.

Using Hewlett's eye-opening research, The Brain Drain Task force, co-founded by Hewlett, Carolyn Buck Luce and Cornel West, persuaded 34 of the world's largest companies to become stakeholders in a task force that works to give women new options and a better shot at promotion.

"Companies that have been quick to change are the ones keeping the best women," says Spence.

Time Warner

(TWX)

, for example, developed a Breakthrough Leadership training program for women with mentors and sponsors helping them to gain a foothold in premier positions.

Lehman Brothers

(LEH)

started the Encore program in 2006, which provides support to women wanting to resume their investment banking career following an extended break.

"I think that in the financial, business school and manufacturing worlds, we'll see all kinds of initiatives popping up in the next year or so," Hewlett predicts, which will open up a new market for on-ramping programs.

Investment banking and law firms have been slowest to change, says Hewlett. "There's still a real entrenched sexism on Wall Street that only smart companies are recognizing and dealing with."

Women have come a long way, but companies still have a long way to go. As pioneering corporations shape a new business model, says Spence, "The companies that don't get it will suffer."