Exercise is safe during pregnancy, but not enough docs know that.

Pregnant women are getting the wrong messages on exercise—often because their doctors cling to old-fashioned ideas.

I was inspired to write about this after reading a post from another PLOS blog, Obesity Panacea, discussing whether exercise is safe for pregnant women. (Short answer: yes.) It’s a topic I’m quite familiar with, having taken a lot of grief about it myself: I ran a marathon while I was pregnant with my second child.

Yes, the kid turned out fine.


I was lucky to be working with midwives who recognize the evidence base supporting exercise in pregnancy, but a 2010 survey of providers suggests that’s a rare situation. Although 99% of the doctors and midwives said that they believed exercise in pregnancy is beneficial, a whopping 64% of them still gave patients the outdated advice to keep their heart rate below 140 beats per minute.

The rule, proposed in 1985, was meant to restrict pregnant women to light to moderate exercise For some of us, a brisk walk gets us up to 140, though it would allow some women to do light jogging. There is, in fact, no reason why women should keep their heart rate below any particular number; the scientist who came up with it explained to ESPN that it was a “guesstimate” based on zero empirical evidence. And so the group that first issued the rule, the American Congress of Obstetrics and Gynecology, removed it from their next set of guidelines.

That means that in the 2010 survey, those providers were giving out a 25-year-old rule that had been retracted since 1994. Worse, 60% of the MD’s in the study had no idea that the rule had been changed.

This is a small taste of what exercising pregnant women are up against.

While exercise is helpful during pregnancy to manage weight gain and blood sugar, and to keep the mom-to-be from going stir crazy, publicizing it as a public health message is sort of unfair when it’s given alongside draconian restrictions. Pregnant women who take to the gym for anything more than a gentle yoga class (full disclosure: I sprained my pelvis in a gentle yoga class) are met with stares and that classic, infuriating question:

“Does your doctor let you do that?”

A doctor, or midwife, doesn’t have the authority to “let” a pregnant woman do something—which also implies that everything is prohibited until the doctor says it’s OK. Rather, evidence should be used to weigh risks and benefits, and ultimately the choice is up to the woman herself.

My midwives asked about my training and race-day plans for hydration and fueling (pregnant women are prone to low blood sugar while exercising, and overheating, while rare, can be dangerous to the fetus) and advised me on some warning signs that could indicate trouble with my pelvic ligaments: if you feel this, stop. Otherwise, they encouraged me to keep exercising as long as I was “listening to my body,” which may sound like woowoo advice but is solidly backed by evidence—absent other problems, pregnant women voluntarily slow down or stop when their temperature rises, for example (thus making obsolete the oft-repeated and probably never-followed advice to stick a thermometer in your vagina mid-workout).

Even though it seems intuitively right, somehow, to advise a pregnant woman that her choices for exercise are walking, yoga, and swimming (or, as I like to put it, “walking gently in a field of pillows”), studies repeatedly fail to find any detriment to mom or baby from even vigorous exercise.

If you can’t imagine wanting to exercise while pregnant, it could be you just don’t have a vivid imagination. I went running 3-4 times per week during the height of my morning sickness, because I knew from experience that I would feel even worse if I stayed in bed. Although I met women who cheered me on, including one who had been out for a run at 40 weeks the day before she delivered her daughter, I had to stop running around 6 or 7 months (shortly after the above picture was taken) but completed a grueling 18-mile hike just so I would have something else to do. Some women can stay motivated to exercise by a goal of “staying healthy for baby” or “training for the birth,” but not all of us. I’m in the camp that needs goals and challenges.

When hiking got too hard, I went to my air-conditioned local gym and lifted weights, including endless kettlebell swings with 35 pounds because that was the biggest bell my gym had, and barbell squats with ever-lightening weight, maybe 85 pounds on a good day. That came to a sudden but temporary stop after I mentioned my routine to a midwife who told me that pregnant women “shouldn’t lift more than 25 pounds.”

Lift 25 pounds how? I said. There’s a big difference between a 25 pound deadlift and a 25 pound bicep curl.

“Just, you shouldn’t lift more than 25 pounds. That’s the rule.”

None of the midwives could tell me where the rule came from, or whether it applied to people who were experienced at weightlifting and were using proper form and breathing appropriately. Finally, one midwife sighed over the phone and said, Look, there are pregnant women who run marathons but it’s not like we recommend that to everybody.

“I ran a marathon back in May,” I told her.

“Oh!” she said. “Oh! I remember you now.” In the end, she agreed that I could go ahead with the same type and intensity of exercise I’d been doing before pregnancy, easing up on the weight depending on how I feel.

I did some further research and found that the 25-pound limit comes from studies of women who had to lift heavy objects at work. The studies found that this correlated with a small increase in risk of miscarriage and preterm birth, which may or may not have been due to the lifting; another school of thought simply holds that the weight you lift while pregnant should be a little bit less than you would lift otherwise. My midwife admitted that while the evidence on cardio exercise (like running) in pregnancy comes from a small pool of research, the evidence on the risks and benefits of lifting is nearly nonexistent.

And so, as in the case of drugs for pregnant women, we need better research. Pregnant women are exercising anyway; researcher James Clapp admits in Exercising Through Your Pregnancy that many of his studies followed women who chose to disregard guidelines, since he couldn’t ethically randomize them to follow the guidelines or not.

Spoiler alert: the babies pretty much all turned out fine.


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