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Serena Williams Got It Right. Exercise Is Healthy During Pregnancy.

Serena Williams won the Australian Open while in her first trimester of pregnancy. A Sherpa woman in her third trimester climbed to Everest Base Camp.1 Paula Radcliffe, who holds the world record in the women’s marathon, ran twice a day in the first five months of her pregnancy. Olympic track athlete Alysia Montano competed in a high-level 800-meter race while 34 weeks pregnant. Pregnancy can feel like one long list of things not to do. But these women are demonstrating that exercise doesn’t need to be on that list — and the science backs them up.2

There’s now plenty of evidence that regular exercise during a normal pregnancy is good for parent3 and baby. According to a new joint guideline from the Society of Obstetricians and Gynecologists of Canada and the Canadian Society for Exercise Physiology, physical activity can lower the risk of excess weight gain, gestational diabetes and postpartum depression in the parent without harming the baby or causing the child health problems later. Although the benefits of exercise during pregnancy are clear, the message can be difficult to convey in a culture that can seem ready to scrutinize a pregnant woman’s every move.

The Canadian guideline (and the U.S. physical activity guidelines that were published earlier this year) advises that people with a normal pregnancy should get at least 150 minutes of moderate physical activity per week. (The American College of Obstetricians and Gynecologists similarly recommends at least 20 to 30 minutes most days of the week.) Despite all the benefits, one study found that only about 23 percent of pregnant people met the 150-minute weekly activity target.

It’s not surprising that pregnancy might lead some people to shy away from exercise. When a woman becomes pregnant, she’s told — by physicians, relatives, friends and society — not to do a whole host of things. Pregnant people are supposed to avoid alcohol, hot tubs, deli meats, certain kinds of cooked and all raw fish, excessive amounts of caffeine, soft cheeses, secondhand smoke, ibuprofen and a bunch of other helpful drugs, and changing cat litter, among other things.4 That can make pregnant people feel as though the health of the unborn child is entirely within their control, if only they would follow the rules. No wonder that according to one national survey of women who had miscarried and their partners, 41 percent believed they’d done something wrong. (This, despite the fact that miscarriages are common and that 60 percent of them are attributable to chromosomal abnormalities in the fetus.)

There’s no evidence that exercise causes miscarriages (although more research is needed on the heaviest exercisers). But pregnancy can bring a vague dread of something going wrong. And that dread may lead people to perceive that doing nothing is less risky than doing something. The authors of the Canadian guideline consider this in part a communication issue. “Concerns over harms have not been substantiated by research and the risks of not engaging in prenatal physical activity have not been adequately emphasized,” they wrote. A study published this year found that obstetricians don’t always adhere to the American College of Obstetricians and Gynecologists’s guidelines; some fail to advise sedentary patients to exercise and incorrectly recommend that heart rates should stay below 140 beats per minute during pregnancy.

To be sure, there hasn’t been enough research to determine whether activity during pregnancy has an upper limit. Williams, Radcliffe and other high-profile athletes have shown that training hard or racing while pregnant is possible. And a study published in September found that elite athletes (national team level) had no more complicated deliveries than non-athletes. But how athletes adjust their training to pregnancy remains mostly an individual decision. A paper published in 2016 concluded that there wasn’t enough information to generalize recommendations for elite athletes. Instead, the authors advised athletes to discuss training plans with their physicians and stay alert to any signs of trouble.

Vigorous exercisers — whether elite or not — can keep it up, although they may have to modify their routine as they progress, in consultation with their physician, the guideline says. A scientific examination of the possible effects of briskly walking a marathon at 39 weeks pregnant — as a woman did in 2011 just hours before giving birth — found that physiologically, it was “no big deal.”

Ultra-distance runner Tracy Beth Hoeg, a sports medicine fellow at the Bodor Clinic in Napa, California, ran 90 to 110 miles per week during her first pregnancy. “It never occurred to me that it would be bad,” she said. She ran at a slower pace than usual. “I felt amazing,” she said. And when she researched the topic, she found no reason not to exercise as she wanted to.

When Hoeg blogged about her running during her first pregnancy, she got a lot of negative feedback. “People were so mean,” she said. “Someone told me I was going to give the baby muscular dystrophy.5 … Someone said I was trying to kill my baby.” By a subsequent pregnancy, during which she ran six marathons, “people were starting to come around,” she said, and the comments weren’t as negative.

Pregnant would-be exercisers could use the inspiration, along with some social support. Pregnant people who don’t exercise cite many of the same reasons that people who aren’t pregnant do — including a lack of time, of resources or of motivation or enjoyment. And if you’re in the life stage of being pregnant, you may also be in the life stage of already having one or more kids, which has been linked to getting less physical activity. Symptoms such as morning sickness, fatigue and back pain can make it difficult to haul yourself to and from work every day, let alone to exercise for exercise’s sake.

“There are definitely times when women feel tired and unwell, and it’s OK to take a couple of days off,” said Margie Davenport, an associate professor of kinesiology, sport and recreation at the University of Alberta and an author of the Canadian guideline. “We encourage that they return to physical activity as soon as possible.” It’s OK to take it slow. The guideline says that women who aren’t engaging in the recommended level of exercise should slowly work up to it. Vigorous daily activities, such as yard work or vacuuming, can count toward the weekly target of 150 minutes. And activity can be accumulated in short bursts of exercise, rather than single sessions.

The Canadian guideline calls exercise “a front-line therapy for reducing the risk of pregnancy complications and enhancing maternal physical and mental health,” but fully implementing it could require a dose of culture change too. If women were lauded for exercising during pregnancy, instead of shamed or second-guessed, they might be more likely to do it.


  1. Because she’s a research subject, her name wasn’t revealed.

  2. There are some conditions that don’t mix well with exercise during pregnancy, including premature labor and uncontrolled Type I diabetes or hypertension, so pregnant people should check first with their doctor to make sure it’s safe for them to be active. And certain activities, including those involving rough physical contact or a danger of falling, aren’t recommended.

  3. Not everyone who gets pregnant identifies as a woman, but the guidelines are based on research on women.

  4. And that’s just a list of recommendations with some evidence behind them — pregnant people are also told, incorrectly, to avoid insect repellant, hair dye and airplane travel.

  5. Muscular dystrophy is a group of inherited diseases and cannot be prevented.

Katherine Hobson is a freelance health and science writer in Brooklyn, New York.