Want to keep mothers in the workforce? Make it possible for them to stay.

Vodafone, the telecommunications giant, announced in March that it was changing its global policies for new mothers. Beginning this year, all women will be offered 16 weeks of paid maternity leave and the ability to work a 30-hour week at full pay for six months after they return. Vodafone made this policy change after it found that 65 percent of the women who left the company following a maternity leave did so within their first year back. The company believes it will retain more talent and grow careers by instituting automatic flexibility for all new mothers.

What Vodafone is creating here is something many women have been told to fear: a "Mommy Track" — a lower-impact, more flexible work schedule.

The Mommy Track concept has been out of favor for years. Critics see it as a way of sidetracking mothers by routing them into lower-paying, dead-end positions. But that's not always true. If done correctly, the Mommy Track is a great idea. And some companies are figuring out how to do it really well.

How the Mommy Track got a bad name

As soon as the idea of the Mommy Track first appeared in 1989, in a Harvard Business Review article, women were told to fear it. Catalyst founder Felice Schwartz was widely criticized for her point in "Management Women and the New Facts of Life" that there were two types of female executives: those who were willing to devote themselves foremost to their careers, and those who were looking to balance family and career for some period of time.

The women looking for balance would take a slightly slower career path, Schwartz wrote. But with part-time work, job sharing, and flexibility, they could be expected to remain in their jobs, rise to middle management, and even reach the same levels as men and women who had not sought such balance. She suggested that the challenge for companies was to provide the right conditions for women to strike this balance. The payoff was talented women at every level of management, including the top.

Schwartz anticipated that her ideas might not be received well by everyone. "We have become so sensitive to charges of sexism and so afraid of confrontation, even litigation, that we rarely say what we know to be true," she wrote.

And indeed, Schwartz was pilloried in the media. Patricia Schroeder, a US representative from Colorado at the time, said the idea of the Mommy Track is "tragic'': "It reinforces the idea, which is so strong in our country, that you can either have a family or a career, but not both, if you're a woman. Of course the business people love it, because it's what they don't feel free to say, and here's a woman saying it for them.'' And so the Mommy Track got a bad name.

Women leak out of corporations at every level

The result of rejecting the Mommy Track for the past quarter-century has been an exodus of highly qualified mothers from the workplace for some period of time, a transition by many women into less-demanding career paths, and a dramatic decline in the trajectory of their earnings.

Here is what we know in 2015:

There is abundant research that shows having more women in the top leadership and governance positions of business and other professions correlates with higher performance for both the business and the economy. At the same time, there is widespread consternation on how to achieve this goal. Progress toward this end has been far slower than almost anyone imagined. In 1989, with women pouring out of the top business and law schools and already earning the majority of the nation's bachelor's degrees, it would have been almost impossible to imagine that a mere 5 percent of Fortune 500 companies would be headed by a women more than 25 years later.

Female talent leaks out of corporations at every stage. A McKinsey report shows that while women are 53 percent of initial hires, by the time they are promoted to managers the proportion drops to 37 percent, then to 26 percent at the vice presidential level, and finally a mere 14 percent at the executive committee level.

Researchers have studied why women take time away from work for their families, and the reality is that most people think it's good for mothers to have some flexibility in their jobs while their children are young. A 2013 Pew report found that almost 85 percent of adults think the ability for a mom of young kids to take time off work or work reduced hours is a desirable thing. According to the Pew report, "Among all adults, only 16% say the ideal situation for a young child is to have a mother who works full time. A plurality of adults (42%) say mothers working part time is ideal, and one-third say it's best for young children if their mothers do not work at all outside of the home."

Women who seek flexible work arrangements often pay a high cost

Workplaces have not adapted to accommodate these desires, though. The price women pay for flexibility is still astronomically high. When women are left to individually work out arrangements with bosses, there begins the journey into the job wilderness leading through a patchwork of job arrangements, professional compromises, and diminished opportunities. This ends not in women having it all, but in them doing it all: part-time, stay at home, job share, and work from home.

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In her book Opting Out: Why Women Really Quit Careers and Head Home, Pamela Stone, a Hunter College sociology professor, found that while women were many times offered flexibility, it was offered as an informal arrangement with their bosses who might change their minds or leave the company. She also found that arrangements like this had a heavy stigma attached to them. Employees feared that taking these flexible options would indicate to their employers that they were not serious about their careers. These women eventually quit their jobs because the options they were offered soon left them with the problems of "marginalization, stigmatization, and career plateauing or mommy-tracking."

Along with frustration and diminished advancement prospects, career breaks have a cost in lifetime earnings. For female MBAs who have taken just 18 months out of the workforce, earnings were 41 percent less than male MBAs. Put that in perspective: a woman might work 40, even 50 years after she receives her degree, yet the break she took for a mere year and a half will impair her earnings for life. The drop-off in earnings happens immediately after she begins a family and is never close to being recovered.

Perhaps even more daunting than the salary and advancement costs of taking time off might be the struggle to return to the workplace. Most professional women who leave the workplace are surprised that they are doing so, as the stay-at-home option was rarely one they had contemplated. As such, they are certain they will return to their chosen profession.

The reality, however, is different. Debora Spar, the president of Barnard College, writes, "Positions disappear; salaries plummet; professional relationships grow stale. And at the end of the day, only 40 percent of women who try to return to full-time professional jobs actually manage to do so. The rest settle into early retirement, or slower paced, lower-ranked jobs." Returning to work is difficult, and returning to work in demanding, male-dominated fields has proven particularly challenging.

The solution: a better Mommy Track

How do we fix this? With a well-designed Mommy Track.

Some of the companies that have been successful at retaining and promoting women have done so with an explicit Mommy Track, although they don't label it as such. Rather than shying away from creating a path for women that involves part-time work or leaves of absence, they have embraced it.

The keys to success are institutionalizing flexibility with a company-wide policy of individualized arrangements for each woman that has support from the CEO. Recognizing that the needs of each family are different has proven a good place to start. Companies who have put in place an off-ramp with a corresponding and clearly marked on-ramp have already reaped the benefits of their efforts in the loyalty and retention of their female professionals.

Years after Schwartz's writing, Sylvia Ann Hewlett and Carolyn Buck Luce argue in the Harvard Business Review for treating women differently: "Employers can no longer pretend that treating women as 'men in skirts' will fix their retention problems. Like it or not, large numbers of highly qualified, committed women need to take time out. The trick is to help them maintain connections that will allow them to come back from that time without being marginalized for the rest of their careers."

Vodafone may be the latest major company to embrace the Mommy Track, but it's not the first. An example of a successful company-wide program is IBM's, which started two decades ago. The tech giant depends on a highly educated workforce and offers its employees an extended leave of absence period in which to work part time and phase back into their full-time jobs.

In extensive follow-up research, IBM found the most compelling case for the Mommy Track: retention. The program allows employees to reduce their workweek to between 20 and 32 hours while earning the same income rate (prorated) and keeping the same job. Employees can stay in this part-time arrangement for up to five years.

When IBM studied the impact of this program on women who had chosen the option of working fewer hours, it found that 59 percent said they would have left IBM for another job with flexibility, and almost a quarter said they would have left the workforce altogether. It can be little surprise that a company that pioneered such programs is one of the few in the Fortune 500 to have a woman at the helm.

The Mommy Track from its very inception was decried as a dead end, as failing to give women the same path to top management as men. But workplace flexibility is a retention solution, said the Families and Work Institute's Anne Weisberg, not an advancement solution. "The first generation of flexibility solutions all focused on the day-to-day job as the unit of measure," she said. "How are we going to make the job fit this person's life? They didn't think about the long term. How are we going to make this career fit this person's life?"

Programs like Mass Career Customization at the consulting firm Deloitte are the next generation, aimed squarely at elevating top female talent. During twice-yearly evaluations, all employees decide to dial up or dial down their careers across four metrics: role, pace, workload, and location/schedule. Sophisticated programs like this offer women flexibility, control over the intensity and direction of their careers, and, crucially, an upward, although sometimes indirect, career path. Twenty years ago, less than 10 percent of partners at Deloitte were women; now that number is 25 percent.

When discussing the pull of family and career, Cathy Engelbert admits that when she was up for partner at Deloitte in the 1990s and pregnant with her first child, she considered leaving the company. But instead she took to "raising my hand when I wanted to so something different, asking for the flexibility I needed at different points in my children's lives and my life." The strategy worked: earlier this year she was named Deloitte's first female CEO.