Throughout my pregnancy last year, until the final days before my son was born, I would go for a daily run (well, more of a waddle) or submerge myself, whale-like, to swim lengths in my local pool. Then I would do yoga or perform arm-strengthening exercises using giant elastic bands. An inner sense told me that staying active would keep me and the unborn bundle healthy; many fellow expectant mums thought the hormones had gone to my head.
It appeared to pay off: I had no morning sickness, I was brimful of energy for the most part, and I gave birth in less time than it takes me to run a marathon. It might all have been coincidence, of course, but new research suggests not. Scientists are now urging pregnant women to get moving, claiming that most are too sedentary for their wellbeing.
In a study of more than 150,000 pregnant women by exercise physiologists at the University of St Louis, it was found that most did not meet even the minimum requirement of daily activity - 30 minutes of moderate exercise on most, if not all, days of the week for healthy expectant mothers with no medical complications. Dr Terry Leet, who led the research, says many women wrongly believe exercise can damage an unborn child by starving it of blood and oxygen. In fact, a woman's heart pumps more blood than normal to ensure the foetus is not deprived when she works out.
"Pregnant women should exercise unless advised otherwise by their physician because of medical or obstetric complications observed during their pregnancy," says Leet. "Beyond the importance for the health of the mother and baby, staying active and flexible aids in the recovery process of childbirth and can also help with postpartum weight maintenance."
If you routinely worked out before pregnancy, Leet says you should continue, avoiding only "contact sports, scuba diving, or other activities that might possibly cause abdominal distress". Even if you did not exercise beforehand, there is no excuse not to start in pregnancy and Leet says the first step should be "moderate, non-weight-bearing activities, such as brisk walking, swimming, or cycling".
This is a far cry from the advice to pregnant women 20 years ago that a gentle stroll was their physical limit and they should cover no more than a mile a day. Now researchers have confirmed more exercise is not only safe, but beneficial, and more evidence is emerging all the time.
Last month, German researchers suggested that jogging while pregnant may enhance brain development of the foetus. Dr Gerd Kempermann of the University Medical School, Berlin, found that the offspring of mice who ran voluntarily during pregnancy developed 40% more neuronal cells in the area of the brain strongly linked to learning and memory processing, than those who did no activity.
There are, understandably, precautions that need to be taken when exercising. Guidelines set by the American College of Gynaecologists (ACOG), which are also followed in the UK, suggest jerky, high-impact movements (which occur in sports such as squash or netball) should be avoided, especially in the second and third trimesters, and activities in low- or high-oxygen environments (skiing and scuba diving) should be left until after birth. Since the body produces extra heat during pregnancy and the temperature of the foetus is one degree higher than the mother's, there is also a danger of overheating when working out in warm weather or stuffy gyms. Wear light clothing, says the ACOG, keep well-hydrated and stay out of hot tubs and steam rooms. Swimming - or any water workout - is great because it helps to control body temperature.
The ACOG says workouts should be limited to 45 minutes, but you should stop if you feel exhausted, dizzy or short of breath. And though exercise further increases your calorie needs - pregnancy increases them by 150 calories a day in the first six months and 300 a day in the final trimester - you don't need to eat for two. High-carbohydrate snacks such as bagels and bananas will make up any deficit, says Louise Sutton, lecturer in health and exercise science at Leeds Metropolitan University.
As the months go by, other, more practical limitations become apparent - such as the need to plot jogging routes with plenty of loo stops. Or to remember that as your bump gets bigger, so might your breasts, which could necessitate more than one supportive sports bra (in my case it was three) to bind your Grand Canyon cleavage in place. Breast growth and the expansion of your midriff also puts strain on the back. Yoga can help, but make sure it is the right sort. Raised levels of the hormone relaxin makes joints more pliable and intense stretching activities, such as yoga, more risky. Personal trainer Matt Roberts stresses that "women who want to do yoga should switch to a specific ante-natal class when they are pregnant as some conventional moves can stress their hyper-mobile joints".
Pregnancy presents different challenges for different women, says sport psychologist Dearbhla McCullough, of Roehampton University, Surrey. "Not everyone has an easy ride," she says. "There may be days, even weeks, when morning sickness, tiredness or breathlessness take a toll so that even a short walk feels impossible. Women shouldn't beat themselves up if that's the case."
If you do feel up to it, the benefits are enormous and not just physical: sitting around applying stretchmark cream and eating for two leaves you listless and with time to dwell on the less appealing sides of childbirth. Studies have also shown that babies born to women who exercise tend to be more alert and are less inclined to be overweight toddlers. So next time you see a celebrity new mum shrink back to shape faster than it takes to change a nappy, you can guess why: she probably never let her gym membership lapse.