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Susana Vera/Reuters

It used to be that any researcher who wanted to study the effects of exercise during pregnancy had to first convince expectant mothers that the workouts would not harm their babies.

Now, it's the other way around.

"The hardest part of our study was dealing with the disappointment of the women who were randomized to the control group, since I was asking them to stay sedentary throughout pregnancy," says Dr. Bradley Price, an obstetrician based in Austin, Tex., and lead author of a recent trial.

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Price's results, published in this month's Medicine & Science in Sports & Exercise, add weight to a growing consensus that a minimum dose of moderate exercise during pregnancy leads to better outcomes. But at the other end of the scale, researchers are still debating how much is too much or too hard – and recent studies offer some suggestive new clues.

The 62 women in Price's study were all non-exercisers when they got pregnant. Starting when they were 12 to 14 weeks pregnant, half of them were assigned to a supervised exercise program that included four sessions a week, each lasting 45 to 60 minutes, and continued until they reached at least 36 weeks. The workouts included step aerobics, walks over hilly terrain and circuits that alternated cardio exercises with weight training.

Not surprisingly, the exercise group gained aerobic fitness and muscular strength. They also had just two cesarean sections compared with 10 in the control group, and returned to normal activities more quickly after delivery. There were no differences in birth size or length of pregnancy.

Based on the results, Price calls the exercise program he assigned, based on recommendations from the American Congress of Obstetricians and Gynecologists, a "minimum effective dose" for women who were not exercising before pregnancy. Women who are already at a high level of fitness, he adds, may benefit from maintaining their greater volume of exercise.

It's the women at the very highest level of fitness, accustomed to gut-churningly vigorous workouts, who may need to exercise caution. Until 1994, ACOG guidelines advised pregnant women to avoid getting their heart rate above 140 beats per minute. That restriction has long since been dropped, but questions linger about whether there is an intensity beyond which the health of the fetus might be compromised.

A Norwegian study published this year recruited six Olympic endurance athletes – cross-country skiers, runners and race-walkers with 21 Olympic and World Championship medals between them – and put them through a series of intense workouts at near-maximal efforts between 23 and 29 weeks of pregnancy.

While most diagnostic signs remained normal, researchers picked up a potential warning sign in two of the women at the highest intensities: For a couple of minutes after completing exercise, fetal heart rate and blood flow through the umbilical cord slowed, before quickly returning to normal with no negative consequences.

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Another new study, by Dr. Linda Szymanski and Dr. Andrew Satin at Johns Hopkins University in Baltimore, picked up similar signs in a study of 45 women who underwent a maximal treadmill test to exhaustion between 28 and 32 weeks of pregnancy. In that study, five of the 15 women who were classified as "most active" before pregnancy exercised hard enough that a brief dip in fetal heart rate and change in umbilical blood flow was observed after the treadmill test, then returning to normal in 21/2 minutes. In both studies, all the women ended up delivering healthy babies with no complications.

"We don't know if this is a meaningful finding in the overall health of the fetus," Szymanski says. "However, it certainly indicates that we need to further study this, and I do believe, in some instances, caution is warranted."

So what does this tell us about an upper threshold for safe exercise during pregnancy? Nothing yet, Szymanski says – there are simply not enough data. All the women who experienced symptoms were exercising at above 90 per cent of their maximum heart rate, which suggests a rough guideline; on the other hand, two-thirds of the women who reached that threshold did not show any effects.

Until further data are available, Szymanski tells her patients to be wary of extremely strenuous exercise, stressing the importance of listening to their bodies and stopping if they do not feel well. "The women I worry about a little bit are the ones who are athletes and have lived by the 'no pain, no gain' philosophy for most of their lives," she says. "I think it's hard to put that mentality on the back burner during pregnancy, but I think they probably should."

For other women, the message remains unambiguous: Stay active during pregnancy. And if you are not already exercising, use your pregnancy as an excuse to start – then stick with it. "Pregnancy is a great time to introduce healthy lifestyle changes," Price says, "because the pregnant woman is motivated to do everything she can to produce a healthy baby."

How to stay fit

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In a recent study, pregnant women who performed the following exercise program reduced their chances of a cesarean delivery and speeded up post-delivery recovery time. The workouts last 45 to 60 minutes and are performed four times a week at "moderate intensity" (12 to 14 on a scale where 20 is the maximum).

DAY 1: Step aerobics

DAY 2: Group walk over hilly terrain

DAY 3: Circuits alternating 1 to 10 minutes of cardio (treadmill, elliptical, stationary bike) with 20 repetitions of strength exercises on a weight machine (upper body, lower body, core), followed by five minutes of stretching

DAY 4: Individual brisk walk

Source: Price et al., Medicine & Science in Sports & Exercise

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Alex Hutchinson blogs about exercise research at His latest book is Which Comes First, Cardio or Weights?

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