Nadja has enjoyed a hugely successful career at Lehman Brothers, the American merchant bank. She is also a perfect example of a highly valued employee who almost resigned because she could not bring herself to ask for the flexibility that she needed to care for her family. She so feared the stigma attached to asking for time out that she came within a hair’s breadth of leaving her job.

For 17 years Nadja juggled the responsibilities of a high-profile career and life as a single mother. When she went on business trips she left a “treasure trail” of notes and jokey gifts around the house so that her son Jason could feel her presence every day. She had relatives to live with them so he would grow up with a sense of family.

But in 2003 he turned 17. His childhood was ending. As he started to talk about university, Nadja was heartsick: “It started to sink in that once Jason left I would lose the dailyness of our life together — and miss it dreadfully. As a single mother and an only child, we had been so close.”

Nadja resolved to make the best of the time she had left with Jason. She would turn the university application process into an excuse to spend time with him. “I developed this vision,” she says. “We would take some road trips across the country and look at a whole bunch of campuses. And in between trips I would just be much more available — to listen and rub his back, to rustle up an impromptu meal or just hang out.”

The plan would need her full-time energy for the better part of a year. So the next Monday morning she walked into her boss’s office to resign. It did not occur to her to ask for leave or for a flexible work arrangement. Jason was not a baby needing childcare or a teenager strung out on drugs; had he been going into rehab rather than to university it might have been easier to ask for help.

“The fact that my son was going away and I felt bereft did not seem to be enough of a reason to get the firm involved,” she says. “I thought I had to quit.”

In the end she was fortunate. Pressed to share the full range of reasons why she wanted to leave, her boss urged her to take a leave of absence but to plan on coming back. Nadja was shocked, thrilled and greatly relieved. For six months she focused exclusively on her son, taking trips, helping with essays. Jason was accepted at the university of his choice and Nadja went back to her job with a new sense of commitment.

The first challenge of the modern age was getting women into the workforce. The second challenge is keeping them there. The overwhelming evidence shows that women want to work — and work hard — but they find it almost impossible to clone the unbroken, competitive model of work created by men. How many women like Nadja are lost to the workforce? I wish I could say that her story is standard stuff. But while the tugs of family continue to interrupt women’s working lives, Nadja ’s outcome is much less familiar. All too often a woman in her situation would have been forced to opt out.

Over the past 18 months there has been an uproar on both sides of the Atlantic over what The New York Times has dubbed the “opt-out” revolution in which large numbers of highly qualified women are dropping out of mainstream careers. The trend has been blamed on everything from policy to individual shortcomings to basic genetic differences. But some of the remedies might lie in a new study by the Hidden Brain Drain taskforce, a group of 19 global corporations including British firms such as BT, BP and Unilever that came together to try to tackle the problem.

For the first time the taskforce has been examining exactly how women’s career patterns work.

It seems that women take “on- ramps” into work and “off-ramps” into career breaks. The study found that some 37% of highly qualified women voluntarily leave their careers for some time. A further 58% take a variety of “scenic routes” (flexible or reduced-hour options): 58% describe their careers after motherhood as non-linear.

“Off-ramping” decisions are often triggered by family responsibilities. Childcare issues loom large but so does caring for the elderly — what has been dubbed the problems of the “sandwich carers” by the British government (24% of women off-ramped because of an elder care crisis). Among women who take off-ramps, the overwhelming majority (93%) really want to return to work. However, only 74% of those who want to rejoin the ranks of the employed manage to do so.

Women off-ramp for a surprisingly short time — 2.2 years on average — but even such relatively short career interruptions entail heavy financial penalties. On average, women lose 18% of their earning power when they take an off-ramp, and the figure rises to 37% when they spend only three years out of work.