Pregnant people have long been told to take it easy. Fearing that hard exercise might hurt the baby, experts set all sorts of limits: “Don’t let your heart rate go over 140 beats per minute.” “Don’t lift more than 20 pounds.” “Don’t exercise for longer than an hour.”
But many of those rules were derived from expert opinion, not science. “They’re based on theoretical risks that may happen—not any empirical evidence,” says Margie Davenport, director of the Program for Pregnancy and Postpartum Health in Alberta, Canada.
For instance, the advice to keep the heart rate under 140 was published in 1985 guidelines from the American College of Obstetricians and Gynecologists (ACOG) because, at the time, experts knew that lower-intensity activity was beneficial but lacked data on harder efforts.
The authors set 140 as an upper limit because it’s roughly the threshold between moderate and vigorous activity, Davenport says. In the absence of research, a conservative approach made sense.
Now, that’s changing. A slew of studies over the past few years—looking at both elite athletes and enthusiastic recreational exercisers—have repeatedly found that pushing the body during healthy pregnancies is not only safe but can be beneficial for both the mom and the baby.
That’s flipping the script for anyone who wants to be active during pregnancy, says Melanie Hayman, an associate professor of health sciences at Central Queensland University who co-authored the Australian Institute of Sport’s 2025 recommendations for pregnant athletes. “It’s no longer about what [pregnant] women shouldn’t be doing, but what women can do,” she says.
Rethinking the limits of prenatal exercise
Across intensity, duration, and strength training, a growing body of research is challenging long-standing assumptions about what the pregnant body can safely handle.
Take the taboo around intense exercise: One 2025 study found that even when participants reached around 90 percent of their max heart rate during a high-intensity interval training (HIIT) workout, there was no evidence of harm to the fetus. Other researchers found that vigorous exercisers had fewer pregnancy complications, such as high blood pressure disorders or gestational diabetes, than those who stuck to light (or no) activity. These benefits may stem from improved cardiovascular function, blood sugar regulation, and placental blood flow.
(Exercise during pregnancy may cut a child’s asthma risk in half.)
Researchers are also revisiting the amount of activity considered safe. Hayman says that some pregnant people do four or more times the recommended 150 minutes of weekly aerobic exercise, and researchers are finding that they’re still just as likely to deliver healthy babies. One 2025 study showed that, among highly active people like marathoners, those who exercised more than 300 minutes a week and longer than an hour at a time in the third trimester actually had reduced odds of delivery complications, including cesarean section.
Strength training is undergoing a similar reassessment. Experts used to advise caution based on data that occupational lifting increases the risk of miscarriage. But Davenport points out that repeatedly lifting heavy objects for hours at a time for work is very different from spending 30 minutes using free weights at the gym, focusing on your form and taking proper rest breaks.
New data reflect that distinction. Recreational weightlifting above 20 pounds is now associated with reduced odds of gestational hypertension and diabetes, mood disorders, and excessive fetal growth. Even lying on your back for lifts like a bench press has been given the green light, despite often-repeated advice not to exercise on your back during pregnancy. Hayman says that while some people might need to modify if, say, they start to get dizzy, there’s no evidence to suggest it’s inherently dangerous.
(No time for daily exercise? Weekend workouts still offer the same health benefits.)
For those who are already active, these findings are easing anxieties that their favorite workouts could be risky. In fact, research increasingly points to the opposite risk: inactivity.
We now know that inactivity is connected to increased odds of pregnancy complications like gestational diabetes and preeclampsia, delivery complications like the need for a cesarean section, prenatal mental health issues like depression and anxiety, and fetal risks like low birth weight and preterm birth.
There’s also post-pregnancy to consider: Pulling back by 50 percent or more in the first and second trimesters is linked to an increased risk of injury postpartum, likely due to detraining. “It’s inactivity that is dangerous and associated with all of those adverse outcomes, not activity,” Hayman says.
That shift in understanding also challenges another common belief—that people who weren’t active before pregnancy shouldn’t start. In fact, emerging evidence suggests the opposite.
“We did specific analyses to identify whether it was safe or beneficial to exercise for the first time during pregnancy and, incredibly, we actually saw even greater benefits compared to those who were active before,” Davenport says.
Why exercise isn’t one-size-fits-all in pregnancy
Of course, there are valid reasons to skip the gym. Someone might have serious medical contraindications or not feel up to working out thanks to pregnancy-related nausea or fatigue. And not every pregnancy calls for high-intensity training or heavy lifting.
“It’s hard for [most of] us to be Serena Williams in a non-pregnancy state—trying to become that during pregnancy is probably not the best idea,” says Cynthia Gyamfi-Bannerman, chair of the obstetrics, gynecology, and reproductive sciences department at the University of California, San Diego, and an author on ACOG’s most recent guidelines. You don’t want to stress out your body to the point that you start feeling pain or intense exhaustion, or experience bleeding or contractions, she says.
Gyamfi-Bannerman advises all patients to talk with their obstetric care provider about what kinds of exercise make sense for their pregnancy. However, there’s now data that shows that conversation doesn’t have to focus on restrictions, but, as Davenport puts it, “the profound benefits of any level of activity.”
@NatGeo
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