most women who don't want children keep a well-stocked pile of contraceptives in the cabinet and hold their breath until the biological clock strikes midnight. But some choose sterilisation. More women under 30 are sterilised in Britain than anywhere in Europe or America - so what makes a healthy, childless woman take such a bold step?

Marie Stopes, a charity and a leading family planning service, is one of the few organisations willing to refer people for sterilisation regardless of age, marital status or whether they have children. Sheila Hancock, sister in charge at Marie Stopes, London, believes it is perfectly possible for a woman in her twenties to make an informed decision, provided she has the right counselling and views sterilisation as irreversible.

Sometimes pregnant women coming to her about termination tell her they have already been turned down for sterilisation. "I do think doctors can be incredibly cautious, even though women at 24 or 25 are quite capable of making up their minds," she says. "Women have to make major decisions in their lives all the time. But if a woman is very young - 21 or 22 - I would certainly feel she should be given another six months to make up her mind."

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The numbers are small, but some do make up their minds early on. The youngest Marie Stopes finally agreed to sterilise, after three long counselling sessions over a year, was 19.

The health writer Jane Bartlett spoke to many childless women for her book Will You Be Mother? and some revealed they had been sterilised at an early age. "A lot of them had kept it secret, particularly from their parents," she says. "What I found was that women who had had it done in their twenties and early thirties knew from an early age they didn't want children, often at 9 or 12. Sterilisation was a way of closing the lid on the issue."

At the back of the minds of some was the fear and risk of abortion, she says, whereas for others it was an insurance policy in relationships. If his partner is sterilised, a man knows up front that babies can never be part of their future and that he can't change her mind later. Among her case studies was a 22-year-old seriously intending sterilisation. She was in a stable, loving relationship, and although she couldn't rule out the possibility that her partner would change his mind about a family, she knew she wouldn't. "She'd tried for sterilisation already and been turned down but she was going to try later," recalls Bartlett. "She was very mature and a real highflier in the civil service. Children weren't part of her future."

A female sterilisation involves blocking the Fallopian tubes between the ovary and the womb with clips, rings or cauterisation (burning). There is "some abdominal discomfort", but usually women can have the operation and be back at work in three or four days. Done privately it costs around pounds 400.

There is no official age criterion for the operation - the decision is made by patient and doctor together. However, most doctors in the NHS and private sector will not consider a woman until at least 25, and are likely to recommend 30. Consultants, understandably, worry that the young girl now sitting in their consulting room confidently persuading them that children are not for her, could in 10 years be the sterilised thirtysomething aching for a baby.

Peter Brinsden, the medical director at Bourn Hall Clinic, Cambridge, who has carried out reversals, believes it is common for women who later regret being sterilised to have had a deep conviction when they were younger that they never wanted children. He believes women should not be considered for sterilisation until at least the age of 28, and says women who do come to regret the decision have not always received adequate counselling before taking what is a "very, very big step".

"I've seen a large number of women who've had a sterilisation at a young age and they've changed their mind," he says. "They've met a partner who wants children and they want to arrange a reversal so they can start a family." He estimates there is a 50-50 chance that a young sterilised woman will regret her decision.

The success rate of reversal is 40-50 per cent under the age of 35, and figures suggest those sterilised under the age of 30 are more likely to regret it than older women. With marriage breakdown at an all-time high in Britain, one of the key reasons women (and men) are likely to seek a reversal is because they want to start a family with a new partner.

These figures don't sway Kate, a 26-year-old married marketing assistant who was turned down by a London clinic over a year ago. "The doctor told me it wasn't necessary when there's such good contraception around and that I could regret it at my age," she says. "But I've got a friend my age who's pregnant, and you can bet no one tells her, 'You'll change your mind.' The way I look at it, having a kid is just as much a life-changing, irreversible decision as sterilisation - or more so."