Illustration: Andrew Dyson.

Illustration: Andrew Dyson.

In the real world, very few women with children can afford to opt out of the rat race for the sake of their families.

The way Sarah Jessica Parker talks about balancing the simultaneous pressures of living and working while spruiking her new film, you'd think I Don't Know How She Does It, could be rebadged The Passion of the Working Mother.

''It is a heroic endeavour every day,'' she told her Melbourne press conference, ''how [working mothers] accomplish the things they accomplish with little or no support.''

In this, SJP is just channelling the exasperated hyperbole of Allison Pearson's comic novel, which struck such a chord in 2003. The book, and the now film, has prompted the choir to preach about the working mother's burden, and the possibility of opting out of the rat race instead. But the reality is that very few mothers can afford the option taken by the novel's heroine, Kate, who quits her job to save her family. And the brutal truth is that even fewer should consider it.

In our anxiety to support a mother's choice to stay at home, we haven't been frank about the consequences.

Opting out of a career for the sake of the children makes the weekly balancing act much easier in the short term, but women making that choice take a terrible risk. They're gambling that their partners will never lose their jobs, never get sick, never die suddenly and never leave them. Every woman needs to remain employable, for her own future and for the sake of the family she might have to support.

This is usually such a middle-class discussion - should nannies be tax deductable? But for women on welfare, particularly single mothers, it's dangerous territory. Unemployed single parents have no chance in private rental. Seven in every 10 single-parent families seeking emergency accommodation are turned away.

Talking to women fleeing domestic violence, I met many de-skilled after years out of the job market. And employers don't want to hire them, even though jobs are the best way to help them.

Time out of the workforce, followed by years of casual or part-time work, extracts a serious financial toll on those who choose it, including the new, stay-at-home dads. Even the modern family ideal, where one spouse has a career and the other works limited hours at a low-key position, has its problems.

The ''mother penalty'' for women with two children is $466,000 in lost income over a three-decade career, according to recent calculations by the University of New England's Professor Michael Bittman. It's not just the years at home, but the pay cuts to fit work hours around schooldays or holidays. Women on reduced hours don't get promoted. They've dodged training, they've shrunk their superannuation.

A study of older women in rental accommodation by Swinburne's Dr Andrea Sharam identified a host of women who never thought it would end up like this: they were tertiary-educated mothers who had spent some years at home and then worked part time. They were doing well, until divorce rewrote the script. When their marriages ended, they were underpaid, under-skilled and undercapitalised.

For many women, the home-versus-work debate is moot; by 2008, more than half of partnered mothers - 52 per cent - were returning to work before their youngest child turned two. More than half of all employers offer some kind of paid maternity leave, and the government now kicks in an extra 12 weeks.

John Howard's child-care rebate persuaded many mothers they could afford to go back to work, and the rising cost of housing made every extra dollar count.

But even if mothers spend all their wage on childcare when they first return to work, it would still be worth it. While employers want women to come back from maternity leave, they're much less likely to hire a mother with young children, according to the 2010 Working Mothers Study by consulting firm Regus. Only 41 per cent of firms planned to hire more mothers, compared to 55 per cent a year ago. Gallingly, half the firms surveyed said mothers returning from maternity leave were assets because they were cheap. Women trying to convert part-time roles back into full-time ones discover their firms won't let them - they're already good value at half price.

It's the career compromises many working mothers make that account for much of the widening pay gap between men and women.

The female-dominated industries that do offer flexible hours, such as community services or retail, are the worst paid sectors in Australia - partly because they are dominated by part-timers, and partly because women's work is not as valued.

But these discouraging statistics are not a reason not to work. They're a reason to campaign for true family flexibility, where employers don't send part-timers down the mummy track, but plan a real career path. It means developing family-friendly practices for all full-time employees. It also means mothers should commit to making their jobs work for others in the office. Yes it makes life more frantic, but working families prove all the time that they can muddle through together.

Indeed, if raising a family stops being career suicide, more fathers may opt for flexible conditions, making it easier for both parents to remain truly employable.