Alma Bennett, the essence of charming, attractive, calm and caring motherhood in Maggie Gee's new novel, Lost Children, cracks apart like a ripe oyster when her beloved teenage daughter, Zoe, runs away from home. This abrupt ending of her role as mother, as the person with an immutable sense of purpose and worth, sends Alma spiralling into crisis.

Her dilemma is typical of that suffered by many middle-aged women. What is it, after years of secure routine, that makes a woman crack? The triggers can be many and varied: children leaving home, or suddenly finding herself peering at a partner who has come to represent utter tedium and wondering, 'Is this it?'.

And then there is the fear of what lies ahead when a woman feels that her sexual shelf-life is rapidly shortening. For those who have not developed careers, there are worries about what to do with the rest of their lives. What often results is an intense desire for change.

Alma's response is to put a bomb under the old certainties. She leaves her husband, emotionally abandons her son and unleashes an anger that she had stifled for years.

On entering the home of Maggie Gee, you cannot help wondering where her character Alma actually came from. Ms Gee's home, in a quiet street in north-west London, is a diminutive place of light, bright wellbeing, with rooms full of colour. And she is a pale and startlingly pretty 45-year-old, who seems to defy all the stereotypes of sagging skin and motorways etched under the eyes.

As Ms Gee talks of a happy marriage to Nick, a radio reporter who 'thinks I am beautiful, so the business of getting to look older isn't an issue for me', her earlier book, Where are the Snows? - about a woman's struggle to cope with the ageing process - comes to mind. Then she describes the delight she takes in her seven-year-old daughter, and her pleasure in writing.

So there is not much sign of chaos or upheaval here. Indeed, Ms Gee seems almost surprised that I think her new book is about a mid- life crisis. 'For me, the genesis of the book was the idea of 'losing' a child,' she says, 'and I started thinking that the lost child, the child who runs away, must in some way be connected to something that is lost or missing about the parent. And I do think that, somehow, people in their late thirties and forties start looking back on their childhoods and their original families in a different way. And sometimes that is an interesting point in the life of their own families.'

Suddenly we are into it. Ms Gee is describing a year in which everything began to fall apart: both her parents died, she developed repetitive strain injury, then near-fatal pneumonia. She may not have chosen to create havoc, but it led her to explore the part of herself that needed attention. 'I became terribly aware of mortality, fearful for my health as I age. I saw myself as having a fragility I had not thought about before.'

Somewhere in all this awfulness, she saw the need to analyse the so-compliant, quiet, eager-to-please- her-parents person she had been right up to her thirties, when she was taking degrees she didn't much want. In her book, when Alma recognises how thoroughly she has been honed by subtle blackmail and threats, she gives rein to her anger.

Ms Gee's soft voice betrays no such signs. 'It took at least 10 years after leaving home before I stopped trying to please. All the time trying to please, and you never can by trying.' But, she says: 'You can't understand anything unless you have tried to understand your childhood.' And you cannot reach a mature understanding 'until you have enough life under your belt'.

Lost childhood returned to Ms Gee in another way, too. She had a miscarriage a few years ago and considers it unlikely that she will conceive again. She passionately describes her deprivation as 'tragic' - nature's cruel denial of something so integral to women, while the oldest of men go on fathering babies.

Ms Gee is keen to talk too about the satisfaction of having become an established writer in mid-life, not having been published until she was in her thirties, and enjoying having a voice through which she can address the things that matter to her. And so it is that Alma's tale is interlaced with a scathing comment on the state of a society that has children living on the streets - another category of lost children.

Then it's back to mid-life - the upbeat side. We have a generation of women who are proving that they can be at least as effective, sensual and vital as they were in earlier decades - and possibly more so with children out of the way and all the time in the world to devote to working hard and having fun. But why, Ms Gee wonders, are women not allowed to simply be admired for this?

She recalls a recent evening when she was out with friends: 'After I had gone, apparently someone said I was looking good, was I on hormone replacement therapy? What does it matter anyway? Why do women always have to be on something if they look good or are full of life? Men don't get treated that way.'

Then there is a thoroughly cheering thought about getting older: 'One of the worst things about being young is that you think everything is your fault, and it's such hard work. The great thing about being middle aged is that you stop thinking everything is about you. You realise that, in fact, practically nothing is. That the reason the ticket collector was so dreadfully rude had nothing to do with your face, it's just that he was having a bad day. And that friend who rang off as soon as you rang was merely having a row with her husband. What a relief it is.'

'Lost Children' is published by Flamingo at pounds 14.99.

(Photograph omitted)