Go-getting women with highpowered jobs and salaries to match might appear to have it all. But results of a new study suggest that, unwittingly, these twenty and thirtysomethings are reducing their chances of having children by doggedly pursuing demanding careers.Professor Elizabeth Cashdan, an anthropologist at the University of Utah, believes that the pressures that come with the superwoman lifestyle are to blame for hormonal and body shape changes that might affect fertility. The result? When it comes to starting a family, many women struggle to conceive.

In her research, published in a recent issue of the journal Current Anthropology, Cashdan found that career women were less likely to be curvaceous with the waspish waist and hourglass shape long associated with fertility and, instead, displayed the more masculine, straight-up-and-down figure that is less conducive to child-bearing.

With work stress and the drive to succeed, Cashdan says, comes a shift in hormonal balance that leads the female hormone, oestrogen, to be replaced by androgens, a class of hormones that includes testosterone and that are associated with strength, stamina and competitiveness.

Crucially, Cashdan says, this appears to affect a woman's waist to hip ratio (WHR), a formula derived by dividing someone's waist circumference by her hip measurement. Classic Marilyn Monroe types with large breasts and narrow waists typically have a curvy WHR of 0.7 (ie, their waist is 70 per cent of their hip circumference) which has been linked in numerous medical papers to optimal fertility. One 2004 study in the journal Proceedings of the Royal Society suggested that women with hourglass figures had about 30 per cent higher levels of the female reproductive hormone, estradiol, compared with other body shapes and that, as a result, they were roughly three times more likely to get pregnant.

Large-breasted and narrow-waisted women have also been shown to have higher levels of oestrogen and progesterone, other female hormones linked to fertility.

But when Cashdan analysed the WHR of women from 37 different populations and cultures, she found their average WHR to be above 0.8 - that's less Sophia Loren or Jessica Alba and more Keira Knightley - a ratio that makes it harder to conceive.

“Although the hormonal profile associated with a high WHR may favour success in some stressful and difficult circumstances where women must work hard, there are well-known costs,” Cashdan says. “Women may suffer lower fertility and possibly lower attractiveness to men who may have an innate preference for curviness.”

Many of Britain's leading infertility experts are unsurprised by the findings. “Certainly, at my clinics we see predominantly very successful businesswomen who do not have hourglass figures,” says Laurence Shaw, associate director of the London Bridge Fertility, Gynaecology and Genetics Centre and a spokesperson for the British Fertility Society (BFS). “They are very slim, very straight-up-and-down.”

Dr Martin Tovee, a psychologist at Newcastle University who has studied the influences on female body shape, says that it is determined by a number of different factors, but that being a go-getter could be one of them. “We know that if women over-exercise or diet obsessively, then their oestrogen levels drop and they become less fertile,” Tovee says. “So, potentially, working hard could affect fertility too.”

Super-skinny women who under-eat to stay that way have long been known to risk compromised fertility. Several years ago, Rose Frisch, an associate professor at Harvard School of Public Health, explained how too few calories and too little body fat triggers a woman's brain to switch off her body's ability to reproduce by gradually restricting the flow of a hormone called leptin.

There is what Frisch has described as a “razor-thin borderline” where a drop of just 3lb can tip a normal-sized woman into infertility without her realising it. She may continue to menstruate, but might not ovulate during her cycle. If body-fat falls much lower, then amenorrhoea occurs when the menstrual cycle simply stops.

However, Cashdan's findings suggest that career women can become infertile even if they don't lose excessive weight. The very androgenic hormones that underly the women's professional mettle are also responsible for transferring fat from hips to waist. And the more fat that settles on her belly, the lower her level of female hormones such as oestrogen. “What is at issue here is not overall body weight, but its distribution,” Laurence says. “The new study suggests that these driven women with high-achieving Type A personalities have a lesser oestrogenic state, which doesn't favour their chances of conceiving.”

The stress of some women's lifestyles also plays a significant role. In research at Emory University's School of Medicine in Atlanta, Professor Sarah Berga of the department of gynaecology and obstetrics, has shown how stress often triggers a cascade of events that result in reduced levels of two hormones crucial for ovulation. Women with hectic jobs on top of busy lives, she says, are most at risk.

In one of her studies, Berga found that women who didn't ovulate had excessive levels of the stress hormone cortisol present in their brain fluid, often due to trying to squeeze in too much work and exercise. “Your brain is hard to fool,” Berga says. “If you are under-eating, overworking and over-exercising, then the hypothalamus - the part of the brain that controls the release of hormones - keeps a running tally of what you are doing.”

Around 20 per cent of women who are infertile have problems with ovulation, and lifestyle factors, including anxiety and stress, are often to blame.

Berga found that talking therapies, such as cognitive behavioural therapy, could reverse stress levels and boost the chances of a woman getting pregnant. But, she stresses, it is not just a case of telling career women to “pull themselves together”.

“To the observer these women actually look very well pulled together,” Berga says. “Many don't report feeling stressed and will even say that everything is just fine. But they may have unrealistic attitudes about themselves and others. Often they think that they can do more work than is realistic, and their sense of worth depends upon their achievement.”

If a woman takes her foot off the pedal, it can certainly help. Laurence says that, ultimately, her sensitivity to oestrogen is determined when her oestrogen receptors are established during puberty. You can't change what you are given, he says, but good nutrition, a balanced exercise plan and fewer hours spent getting stressed can pay off.

“Oestrogen makes someone relaxed, calm and thoughtful, the perfect state in which to become pregnant,” he says. “It is no biological mystery that so many studies have shown men are drawn to women who are curvaceous and have a narrow waist, indications of health and fertility.”

Vital statistics

A study of 6,000 women by researchers at Northern Carolina State University found that just 8 per cent of females now have the sort of hourglass figure flaunted by 1950s film stars

A 2007 survey of 9,000 British men and women at University College London (UCL) revealed that, since 1951, the average bust size has increased by 2in to 39in and hips by a further 2in to 41in. However, waistlines have increased a massive 6.8in, meaning that women no longer go in in the middle.

Last year, Dr Devendra Singh from the University of Texas analysed 345,000 texts to confirm that the Western male's preference for a slim-waisted woman has been generally constant throughout the centuries A 2004 survey by researchers at UCL and the London College of Fashion showed that 20 per cent of women now have a pear-shaped figure (with hips larger than their bust), whereas 46 per cent are cylindrical or straight-up-and-down.