Stretch marks, weight gain and overwhelming fatigue are the side-effects of pregnancy with which most women are familiar. But for some, motherhood appears to leave the female body better able to cope with extreme physical demands than ever before. Kim Clijsters, the Belgian tennis player, became the latest elite sportswoman to add credence to the theory that, far from signalling the end of a woman’s athletic career, pregnancy can enhance her performance. Clijsters took a two-year break from tennis to give birth to her daughter but, at the US Open last week, she defied expectation to trounce Venus Williams, the No 3 seed.

She is certainly not the first high-profile athlete to discover that becoming a mother somehow spurs an already high-achieving body on to even greater things. Among those to have experienced the “‘motherhood effect” are the distance runners Paula Radcliffe and Liz McColgan and Catriona Matthew, the Scottish golfer who won this year’s women’s British Open ten weeks after giving birth. All claim that the demands of pregnancy and childbirth made them stronger of body and more wilful of mind, suggesting that in some way the rigours of the process heightened their athletic powers. But beyond the anecdotal, is there any evidence that such a boost takes place?

Medical experts are in little doubt that hormonal and other changes in pregnancy impact on physical performance during the pregnancy itself. In the first three months it is known that a woman’s body produces a natural surplus of red blood cells, the type that are rich in oxygen-carrying haemoglobin, to support the growing foetus. James Pivarnik, a professor of kinesiology and epidemiology at Michigan State University, has studied athletes during and after pregnancy at his Human Energy Research laboratory and found there is a 60 per cent increase in blood volume and that this could improve the body’s ability to carry oxygen to the muscles by up to 30 per cent.

“This could improve aerobic capacity, enabling a woman to run, cycle or swim at a certain pace for longer,” says Greg Whyte, professor of applied sport and exercise science at Liverpool John Moores University. But there are other adaptations in the first trimester that could also make a difference: “A surge in hormones — predominantly progesterone and oestrogen, but also the male hormones including testosterone — could increase muscle strength,” Whyte says. “Increases in other hormones like relaxin, which loosens the hip joints to prepare a woman for birth, could also improve joint mobility to a beneficial degree.”

There are rumours that in the 1970s and 1980s, such physiological improvements during pregnancy led to attempts by East German athletes to enhance their performance by getting pregnant and then having an abortion. In what has been dubbed the “abortion doping scandal”, it is alleged that female athletes fell pregnant for three months in order to benefit from the increased oxygen supplies. The issue was raised by the former chairman of the International Olympic Committee’s medical commission at the first anti-doping conference in 1988 although evidence that it occurred has never been substantiated.

“It is certainly viable that pregnancies were enforced and then terminated as part of the old East German regime,” Whyte says. “Some doctors have claimed they know that is the case.”

Liz McColgan, now a mother of five, ran five miles the day before giving birth to her first child in 1990: “I remember I felt a little heavy that day and didn’t feel right.” She also claims that her first training runs after giving birth left her feeling elated. “You feel really good, comfy and strong because you have got rid of all that weight,” she says. “I felt fantastic after carrying the baby.” After six and a half weeks, McColgan took part in a 5km road race in America and, after 11 weeks, she won a bronze medal in the World Cross Country Championships. “Taking time away from racing while you are expecting also helps,” she says. “It rekindles your love for the sport so that you come back with more passion for it and refreshed.”

Beyond the the pregnancy itself, opinion is divided. Pivarnik believes many of the benefits of pregnancy are fleeting. “I would not go so far as to say that pregnancy is an ergogenic aid,” he says. “But a woman’s enhanced blood volume from pregnancy would possibly help her in a race performed two to three weeks postpartum and many women who continue training perform very well after having a baby.”

But Dr David James, a researcher in exercise physiology at the University of Gloucester’s faculty of sport, says that the effects of pregnancy on performance are notoriously difficult to study. “Neither researchers nor pregnant women are enthusiastic about participating in studies in which they could possibly endanger babies,” he says. “Consequently, there are few confirmed findings on the subject.”

But results of studies that have been published suggest childbirth is beneficial to sportswomen. A 1991 analysis of recreational runners revealed that the efficiency with which the body uses and processes oxygen increased by 7 per cent in the eight months after childbirth. And researchers reporting in the Scandinavian Journal of Medicine and Science in Sports in 1997 revealed that 11 per cent of elite endurance performers, such as cross-country skiers and runners, performed significantly better after having a baby while 61 per cent returned to compete at the same level at which they had performed before pregnancy. Dr Jorma Penttinen of the department of obstetrics and gynaecology at the University Hospital of Kuopi in Finland, concluded that each woman’s body responds differently to pregnancy but that it is unlikely to have a negative impact on their athletic comeback.

“It is also accepted that carefully planned training can be continued throughout pregnancy whereas it used to be considered risky to mother and unborn child,” Dr David James says. “That can make a difference to a woman’s fitness levels when she returns to competition.”

Indeed, while the advice to pregnant women 30 years ago was to do no more than a gentle stroll and cover a maximum of a mile a day, researchers have now shown that exercise throughout pregnancy benefits mother and child. German researchers reported that running while pregnant can enhance development of the foetus and other studies have shown that babies born to active women tend to be more alert and are less inclined to be overweight toddlers.

But perhaps the greatest benefits to athletic mothers are the psychological changes that come from experiencing labour itself. Ingrid Kristiansen, the former marathon runner from Norway, confirms the hunch of many sports scientists in her belief that childbirth aided her sporting success by raising her pain threshold.

Whyte says: “Women re-evaluate where they can anchor pain and many psychologists believe that woman’s pain threshold is effectively reset so that when she resumes or takes up training again, nothing ever seems as uncomfortable.”