The Americans have a name for it — “off-ramps” and “on-ramps” — the process by which well-qualified women either give up work completely, or cut back their hours to care for children or relatives, and then, despite the broken work experience, part-time hours, drop in income and reduced chance of promotion, try to get back into professional life. Among British working mothers, it is known euphemistically as the “mummy track”.

As the partner of Tony Blair’s press secretary, Alastair Campbell, a political journalist and aide to the Blairs in No 10, I have tried every kind of work — full-time, part-time, flexitime. Throughout, I have had crashes of confidence, been permanently exhausted and moderated every career choice with the anxiety that I would let my children down in some way. But it was coming back into the office after periods of working at home that I found hardest to negotiate — the “on-ramp”.

One recent UK study published in The Economic Journal confirmed that the majority of new mothers who remain in work don’t merely switch to part-time hours, but “trade down” as a result. Women in senior positions are particularly badly affected because of the shortage of high-quality part-time management jobs. The study’s authors, Dr Sara Connolly and Dr Mary Gregory, painted a bleak picture. From corporate manager to office worker; from teacher to classroom aide; from nurse to care assistant. These are the occupational trajectories for highly qualified women when they switch to part-time work and childcare.

Estimates suggest our failure to keep well-qualified mothers in the jobs for which they are trained costs between £15 billion and £23 billion a year in lost tax receipts and wasted investment in education. More important, it is an emotional seesaw for women. The conflict between earning money, wanting to work for personal fulfilment and being a “good mother” remains as powerful as ever.

Some women do hang on to the same full-time jobs they had before they became pregnant, but the majority make some sort of change. Beverly, a scientist with a PhD, gave up her job when her first child was born, suffered postnatal depression and started to begrudge her husband his freedom. Eventually, she found her way back into work, but as an academic research administrator. “It’s not what I’m trained to do. It does bother me, but I’m getting something else out of it — time with my children.”

Louise, a former television producer in her early forties who gave up work to raise her two children, says: “I don’t resent my kids, but I do resent the situation. We were brought up to work, told to get out there and do it on our terms, but when I had my children, I realised that wasn’t achievable. I’ve sacrificed work life for family life.”

All parents with children under six can now ask to work flexibly. From next month, so can the parents of teenagers. But the key to keeping more women in employment, on their own terms, is more high-quality part-time jobs, and high-profile role models — a job share in the Cabinet, for example. It was a great week for fiftysomething women when Hillary Clinton became US Secretary of State, and Yahoo! took on Carol Bartz, 60, as CEO. But these high-level appointments are remarkable because they are unusual.

Getting women back to work involves not just jobs but emotional support. Karen Mattison, co-founder of the London-based social enterprise Women Like Us, which seeks to match mothers to quality part-time jobs, says: “We realised we needed to support women on the journey back to work, give them coaching, confidence and the skills they were lacking.”

But getting back on the on-ramp is possible. One mother, Sally, gave up journalism to care for her three children, but became a school governor and was actively involved in local politics during her time out. She used this to good effect on her CV and eventually landed a job campaigning for a national charity. “Anything is possible now,” she told me.

The Secret World of the Working Mother: Juggling Work, Kids and Sanity by Fiona Millar (£12.99 Vermilion) is out now

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