THERE'S NEVER BEEN a better time for women in the western world to be pregnant - technology has eliminated many risks to mothers and their unborn children. But there are still plenty of myths and much controversy about whether and how much a woman can exercise when she's pregnant. On one hand, there are reports about women running marathons while well into their pregnancies. On the other hand, many family, friends and well-wishers advise refraining from exercising until after the baby's born. In Roman times, exercise was considered to enhance people's health, generally - moderate activity was encouraged during the second trimester, and low intensity exercise during the first and third trimesters. Whereas, in the Victorian era in Britain, any form of physical activity by pregnant women was frowned on. Today, the interest in exercise and the rising obesity rates have encouraged research into the effects of aerobic exercise on mother and fetus. As a result, bodies such as the American College of Obstetricians and Gynaecologists (ACOG) have taken a more liberal stance about exercise during pregnancy. 'In the absence of either medical or obstetric complications, 30 minutes or more of moderate exercise a day on most, if not all, days of the week is recommended for most pregnant women,' the ACOG recommended in a report in January 2002. Two year earlier, the US Department of Health and Human Services said that the benefits of exercise - such as weight control, mood improvement and decreasing discomfort and fatigue - could also help pregnant women. Although pregnancy is a time when women gain a significant amount of weight, doctors no longer encourage them to gain any more weight than is necessary to ensure a healthy baby at birth. A study by US researchers in 1990 found that the effect of weight gain on the size of the fetus decreased after a certain point. Researchers from the Gundersen Lutheran Medical Centre in Wisconsin found that women who gained more than the recommended weight - and failed to lose it by the time their baby was six months old - had a greater chance of developing weight problems later in life. Exercise is one of the best ways to avoid unnecessary weight gain. But the researchers found that exercise also had psychological benefits, such as helping pregnant women to cope with anxiety or depression. Other studies - such as one by Wallace and Associates published in the Journal of Nurse and Midwife in 1986 - found that exercise also eased pregnancy problems such as backaches, headaches and fatigue. But although many women say that being physically fit ensures an easier labour and delivery, there's no guarantee. In a study titled Olympic participation by Women: Effects on Pregnancy and Childbirth, doctors found that the strong abdominal muscles of well-conditioned female athletes helped shorten the second stage of labour - but the rigidity of their uteruses and strong muscle tone made for a more difficult first stage. However, exercise usually ensures a more positive mindset, which can be crucial in easing labour. Doctors say that many of the risks of exercising when pregnant occur because of changes to the woman's muscular and skeletal structures. 'The further along the mother is, the more pronounced the curve in her lower back can be,' says Nahed Ezmerli from the George Washington University Medical Centre. If a woman's pelvis ends up having too much of a forward tilt, it can cause her upper back and shoulders to round forward, making her feel unbalanced. When this is coupled with an expanding body and loose ligaments and joints from the hormone relaxin, weight-bearing exercise can be dangerous. However, some of the changes to a woman's cardiovascular and respiratory systems can make exercise easier - at least until the 20th week of gestation. Ezmerli says the amount of blood a pregnant woman's heart pumps can increase by up to 50 per cent. Although the enlarging uterus causes her chest to expand and shorten - often making her feel breathless - her ability to take in and use oxygen can improve by as much as 30 per cent. Regardless of the personal benefits and risks, most pregnant women are 'heavily motivated to do the right thing by the baby', says Robin Kerr, a sports and women's health physiotherapist working in Hong Kong. So, most concerns about exercise relate to what effect there may be on the fetus. One of the more significant physiological changes in a mother's body during exercise is the redirection of blood flow. The working muscles demand more oxygenated blood leaving less for other organs such as the uterus. This decrease in blood to the uterus causes some mothers concern because of the possibility of an inadequate oxygen supply. But a US study in 1976 of pregnant ewes found that the uterus would have to be denied more than 50 per cent of normal blood flow to cause fetal damage. And according to US research in 1979 reported in the Journal of Applied Physiology, fitness is the key: the amount of blood taken from the uterus of a fit mother is much less during exercise than that for an unfit mother. Some mothers are also concerned about possible fetal heart stress brought on by exercise. But most of the research done in the past 10 years shows that a rise in the unborn child's heart rate isn't detrimental to its development or health. A US study in 1996 published in the American Journal of Obstetrics and Gynaecology examined fetal heart rates during exercise by women who were eight months pregnant. It found that the average increase was an inconsequential five beats per minute. Hyperthermia (or overheating) is a more realistic potential risk because of its association with fetal abnormalities. However, many experts say that mechanisms such as an increase in the mother's blood volume and skin temperature help transfer heat from the fetus. One study published in Obstetrics and Gynaecology in 1985 took the temperatures of women who ran five to six kilometres five times a week in the 12th, 24th and 32nd week of gestation. Although their skin temperature increased, their core temperatures never exceeded 39 degrees Celsius. And all gave birth to normal infants. Even so, most health experts don't recommend that pregnant women work out at more than 150 beats per minute or for long periods. The ACOG recommends no more than 20 minutes at moderate intensity because the point of aerobic exercise during pregnancy is for health, rather than increasing sports performance. In fact, intense exercise has been shown to result in low birth weights. A report by James Clapp in the European Journal of Obstetrics, Gynaecology and Reproductive Biology found that 'maintaining a rigorous exercise regime throughout pregnancy selectively reduces growth of the fetal organ and size at birth'. By contrast, moderate exercise didn't adversely effect the baby's birth weight. The most important point for pregnant women is to discuss the issue of exercise with their doctors. A gynaecologist or obstetrician is in the best position to advise what's healthy and what's a risk.