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When Margie Davenport was pregnant with her first child four years ago, a well-meaning nurse advised her to make sure her heart rate stayed below 140 beats per minute for the duration of her pregnancy – a familiar, but outdated recommendation that dates back to the 1980s.

“I was like, ‘Oh, you’re talking to the wrong person,’ ” Davenport joked at a conference last month where she was unveiling the newly formulated Canadian Guideline for Physical Activity throughout Pregnancy.

Now a professor at the University of Alberta, Davenport co-led a team that combed through 27,000 abstracts and reviewed 3,000 journal articles to formulate new guidelines for the Society of Obstetricians and Gynaecologists of Canada and the Canadian Society for Exercise Physiology. Exercise during pregnancy, the authors conclude, should no longer be viewed as an acceptable risk or a nice idea. Instead, it’s a crucial part of a healthy pregnancy.

The key recommendations are that pregnant women should aim for at least 150 minutes of moderate exercise per week, spread over at least three days a week, including a variety of aerobic and resistance-training activities. Hitting these goals, the accumulated evidence suggests, reduces the risk of complications such as gestational diabetes, high blood pressure and pre-eclampsia by about 40 per cent, and reduces the risk of depression by a staggering 67 per cent.

Those are big numbers, and the most important shifts in the new recommendations involve broadening the scope of who should be seeking these benefits and when. For example, the recommendations now apply throughout pregnancy from conception to delivery, since there’s no evidence of increased risk (and some evidence of fewer complications) for women who exercise during the first trimester. They also apply to previously inactive women and to older mothers.

The dose suggestions have also been tweaked. While previous recommendations stuck to three or four days a week, the researchers found the data suggested that more is better. The guidelines now suggest a minimum of three days a week, but “being active every day is encouraged.”

Just as important are the potential reasons not to exercise. Certain conditions like pre-eclampsia and ruptured membranes are considered “absolute contraindications” to exercise – but even in those cases, Davenport notes, women may still be able to continue with their usual activities of daily living rather than being confined to bed rest.

Other situations, such as being more than 28 weeks pregnant with twins, are considered “relative contraindications,” meaning that the pros and cons of exercise should be discussed with a health-care provider. Having a history of pregnancy loss was once on this list; now it’s only considered a relative contraindication if it’s a recurrent history of pregnancy loss.

That doesn’t mean there are no remaining caveats. Earlier this year, Davenport published a case study of a 28-year-old Sherpa woman who was working as a guide on a trek to Everest Base Camp, ascending to 5,300 metres above sea level while doubling the weekly exercise recommendations each day, all while 31 weeks pregnant.

The woman later delivered a healthy baby with no complications – but Davenport was nonetheless taken aback by the headline in the New York Times when the case study was published: “Vigorous Exercise, Even A Trek Up Everest, May Be Safe During Pregnancy.”

Not so for most people, Davenport says. Women who don’t already live at high altitudes should avoid exercise above 2,500 metres, where oxygen levels are sparse, as well as exercise in extreme heat, activities with risk of falling, and scuba diving.

The trickiest question is how to advise athletes who wish to continue at very high intensities. A study of 130 pregnant elite athletes in Iceland published in September found no links to delivery complications. But given the relative dearth of evidence, Davenport remains cautious. Those who wish to do significantly more than the guidelines suggest should talk to their health-care provider, she says – and be flexible.

“During pregnancy, it's especially important to listen to your body, watch for fetal movements, and maintain hydration and nutrition throughout exercise,” she says.

During her own pregnancy, despite the nurse’s friendly advice, Davenport jogged for the first six months and then switched to uphill treadmill walking to keep her heart rate up once jogging became too uncomfortable. As a former national-team synchronized swimmer, it was an easy decision: “At that time, keeping to 140 beats a minute would have caused more psychological stress than not.”

Alex Hutchinson is the author of Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance. Follow him on Twitter @sweatscience.

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