This week's revelation that women's fertility starts declining at age 27 confirms fears, outlined in a new book, that career women may be postponing babies till it's too late, writes Kathryn Holmquist

Young women are planning their careers and personal lives in the belief that they will still be able to have babies in their late 30s and even 40s.

Postpone childbearing as long as you can and there will be "a huge payoff in the workplace", says Felice Schwartz, founder of Catalyst, a British think-tank that promotes equality of opportunity in the workplace.

Lisa Berenson, former editor-in-chief of Working Woman and Working Mother magazines, says that the work ethic in the US is such that "a serious person needs to commit to her career in her 20s and devote all her energies to her job for at least 10 years if she is to be successful". But new research has exposed such optimistic delaying tactics as a game of roulette which is against nature and will leave many women and men devastated by infertility, according to a European study published this week in the journal, Human Reproduction.

Women's fertility begins to decline at age 27, on average, report the National Institutes of Environmental Health Sciences in North Carolina and Padua, Italy. Men should be wary of delaying fatherhood past the age of 35 when their fertility starts to fade, according to the study, which analysed 782 women aged 19 to 39 - and their partners - in six countries. The chances of conceiving during the six-day fertile period which occurs each month is 50 per cent for 19-26 year-old women, 40 per cent for 27- to 34-year-olds and 29 per cent for 35- to 39-year-olds.

What the results mean is that the average 35-year-old woman is still likely to conceive eventually, but it will take her longer than it would have when she was in her 20s. If she waits until her mid-30s to discover fertility problems, it may be too late to conceive even with treatment. With IVF, the chance of conceiving in a single IVF cycle is 8 per cent at age 39, dropping to 3 per cent at 44.

So how do women square their need to succeed in the workplace with their need to procreate? This is a case of society's expectations arrogantly denying nature's realities, so that family life suffers as a result. Nature, it seems, wants women to have babies in their 20s, but society expects women to career-build in their 20s and 30s - the very years in which they are physically most able to have babies and rear families.

A few extremists may label women struggling for equality as "feminazis", but the truth is that far from destroying life, many financially independent women are being prevented from creating it by their employers who insist that women behave like cloned men. More and more high-achieving women in careers who want to have babies with men they love, are finding that work is draining away their nesting years. When women achieve the peak of career-building in their mid- to late-30s and are ready to start nest-building, it's often too late to have a family.

Increasing numbers of successful women are being forced into childlessness by the demands of the workplace, says Sylvia Ann Hewlett, economist and author. Her latest book, Baby Hunger: The New Battle for Motherhood caused consternation in certain circles in the US.

A political adviser to Bill Clinton and Neil Kinnock, Hewlett is currently chair of the US National Parenting Association and lives in New York City with her husband and children - the last of whom she had at the age of 51 (with the help of IVF). Young women shouldn't follow her example, if they want the best chance of becoming mothers.

Depending on which study you read, between 39 per cent and 61 per cent of high-achieving women are childless at mid-life, even though the majority of these always expected to have children eventually. (Her definition of high-achieving is earning more than €48,500 a year.)

Hewlett exposes high-achieving women as having lived in a fantasy land of unrealistic expectations. Nine out of 10 young women believe they can become pregnant in their 40s, according to her ground-breaking study, conducted in association with Harris Interactive and the National Parenting Association in January 2001. Yet these women are extremely unlikely to even find a partner and marry after the age of 35, much less have babies.

The statistics are harsh: 55 per cent of 35-year-old career women are childless and are likely to stay that way. The possibility of combining career and family has decreased during the past 20 years - showing that women have been failing in their struggle for equality. Only 38 per cent of 35-year-old career women were childless in the previous generation.

Men don't have this problem: 57 per cent of high-achieving women are married, compared with 83 per cent of high-achieving men. Only 25 per cent of high-achieving men are childless at ages 41-55. But 33 per cent of high-achieving women aged 41-55 are childless, and this figure rises to 42 per cent of women in corporate careers and 43 per cent of women in academic careers.

Eighty-six per cent of these women don't want to be childless - it just happened that way. One of those quoted in Hewlett's book calls putting off nesting until it's too late "a creeping non-choice".

Dropping  fertility is only part of the problem. Hewlett's research shows that women want to create sons and daughters not on their own, but in loving relationships with men. Yet relationships require what is now a luxury: private time. Two-thirds of high achievers work more than 50 hours per week and 13-hour days (including commuting time).

There is also the issue of power in relationships. Hewlett writes of women who have discovered that many "A+" men prefer "A-" women who will not compete with them. Tamara Adler (43), managing director of Deutsche Bank in London, told Hewlett that "in the rarefied upper reaches of high-altitude careers, where the air is thin and it's hard to breathe, men have a much easier time finding oxygen. They find oxygen in the form of women who will coddle their egos and make them feel like a king or some other kind of superior being."

Men in their 30s or 40s who feel the biological urge to nest can mate with fertile women in their 20s who are willing to put their careers on the back-burner. It would be much more difficult for women to find the equivalent.

Hewlett's research has exposed the reality: a woman's best chance of having a family comes when she finds a man to be financially dependent upon and can give up work. Her second best chance, Hewlett's survey found, is if she is an entrepreneur who can determine her own lifestyle and has her children young. Also well-placed for career success and family are those women so talented that they can work hard, make themselves indispensable and then demand family-friendly working arrangements from their employers. But such lucky women are rare.

The ordinary woman who is committed to succeeding in the corporate or academic world, may find herself sacrificing her fertility, whether knowingly or not. Even if she succeeds in having children in her 30s, she will be severely penalised, either in her career or in her personal life. It's a Catch 22 situation: plan your life around babies and your career will suffer; plan your life around your career and there may be no babies.

Having it all? There's rarely such a thing. Hewlett believes women should be creating backwards career plans, choosing where they want to be at 45 and then counting back to create a schedule of how they'll achieve it - marriage at 30, first child at 34, second at 36, third at 38.

Men don't have to think this way. "High-achieving men do not experience a significant gap between what they want and what they have on the children front," writes Hewlett. "70 per cent wanted children, 75 per cent have children. This doesn't mean life is easy for fathers. Only 7 per cent, in Hewlett's survey, believed men could "have it all". They had left child-rearing to their wives and had sacrificed knowing their children in order to earn money.

Hewlett's response is snippy: "At least they can enjoy their grandchildren". A deeper view would be that men too have something to gain from making career success less hostile to family life. As for childless workers who don't want children: they too should be allowed private time and should not be forced to compensate for workers with families.

Baby Hunger: The New Battle for Motherhood by Sylvia Ann Hewlett is published by Atlantic Books. £10 sterling in UK.