Endurance is a defining feature of pregnancy, as these ultra-athletes know

World-class athletes like Allyson Felix are redefining the limits of what’s possible while pregnant and postpartum. (Steph Chambers/Getty Images via CNN Newsource)

Two-time track and field Olympian Molly Huddle was pregnant past her May due date. Since she’d been running throughout her pregnancy, she turned to what she knew best: “Trying to ‘sprint the baby out,’” she joked on Instagram, tagging it #speedwaddle.

Three days later, on May 28, her second child, baby Louisa, arrived.

Huddle had been there before. Both she and fellow pro runner Kellyn Taylor had returned to racing after giving birth to their first children, entering the New York City Marathon in 2023. Taylor finished first among American women, averaging an impressive 5-minute, 43-second mile 10 months postpartum. Huddle followed close behind, placing second.

A year before that, Chelsea Sodaro won the Ironman World Championship in Kona, Hawaii, just 18 months after having her daughter. She was the first American woman to earn the title in 25 years, the first newcomer to do it since 2007, and the first-ever new mom to win.

These women aren’t outliers anymore. They are at the leading edge of a growing movement of athletes who are redefining the limits of what’s possible while pregnant, postpartum and nursing. And this shift is being acknowledged at the highest levels of sport, like the addition of the first-ever nursery and breastfeeding spaces at the 2024 Paris Olympics.

Huddle ran throughout her pregnancy, as did Taylor, who completed a 5K race when she was 32 weeks pregnant. They are part of a broader wave of athletes rejecting decades of outdated guidance that urged pregnant women — regardless of fitness or risk profile — to scale back intensity and duration of exercise.

Challenging pregnancy norms

When these women and those before them broke with previous recommendations to reduce their physical activity while pregnant, it enabled researchers to argue for the necessary science to back them up.

Most prior recommendations for pregnancy weren’t based on strong science, said Dr. Emily Kraus, who leads Stanford University’s FASTR (Female Athlete Science and Translational Research) program.

“Because of all the ethical challenges with doing actual studies on pregnant women, many of these past guidelines weren’t grounded in research studies, but rather doctors’ best judgment,” Kraus added in an email.

Now, science is finally catching up to the athletes who pushed the limits.

A landmark 2022 study by a team of Canadian researchers tracked 42 elite to world-class distance runners through pregnancy and the postpartum period. These athletes maintained running volumes two to four times greater than what current exercise guidelines recommend for pregnancy.

On average, those runners returned to training within six weeks of giving birth and reached 80% of their prepregnancy training volumes by three months. More than half performed better after giving birth than they did before. For those aiming to return to elite competition, their race times remained statistically unchanged.

“What we’re finding is that there’s improvements in health outcomes when you maintain your training levels. In addition, we’re also seeing a reduction in injury risk in the postpartum period,” said Margie Davenport, a professor of exercise physiology at the University of Alberta, of her latest research.

Davenport found her data all pointing in the same direction: If you are used to it, hard fitness training during pregnancy can be beneficial, and that includes activities that were previously not recommended, such as contact sports and heavy strength training, both of which she has studied. “Unless, of course, you develop a contraindication or a medical reason why you shouldn’t, or if you just choose to, which I think is entirely everybody’s prerogative,” Davenport said.

Endurance is a defining feature of pregnancy, which places sustained, monthslong demands on the body’s cardiovascular, muscular and metabolic systems. As it turns out, ultrarunning has a similar impact. A 2019 study found that the physiological ceiling for sustained energy expenditure — the limit seen in events like the Tour de France or the Ironman — is about 2.5 times a person’s resting metabolic rate. The average pregnant woman sustains nearly that (2.2 times resting rate) for weeks on end. Add in breastfeeding and the body’s metabolic investment climbs even higher.

“The physiological changes that accompany pregnancy mirror quite closely — with only a couple of exceptions — endurance-training changes that all bodies experience,” said Cara Ocobock, a coauthor of the study and now a biological anthropologist and associate professor at the University of Notre Dame in Indiana.

Pregnancy is a feat of athleticism all on its own, and the suite of metabolic processes that make it possible may be why female bodies have powerful advantages in endurance.

Keeping up with pregnant athletes

Canada, the United Kingdom and Australia have recently updated their recommendations for pregnant women, with significantly more activity suggested than before as well as much greater latitude for athletes. In the United States, the American College of Obstetricians and Gynecologists advises at least 150 minutes a week of moderate-intensity exercise. The group also recommends reducing intensity by keeping heart rates below 140 beats per minute, lower than what is typical for many athletes.

For pregnant athletes, this level of activity is “essentially detraining them,” Davenport said.

Conversations around pregnancy and activity still focus on limitation and risk, athletes say.

“Patients can get very different information about what to do during pregnancy and postpartum, based on the obstetrician’s comfort level and familiarity with these studies,” Kraus said. Unfortunately, she added, there can be a real “culture of fear” around exertion during pregnancy.

Shefali Christopher, a clinical specialist in sports physical therapy and associate professor at Tufts University, recalls that lack of advice during her own first pregnancy. Although she had been doing the challenging training necessary for Ironman competition, the shortage of clear, evidence-based guidance led her to err on the side of caution and become sedentary.

When she was ready to start training again after having the baby, “I got really terrible advice from health care providers. I was told to go ahead and exercise without any guidance on frequency or duration or anything,” she said. Recovery was tough. That’s when she dedicated her own doctoral research, and her work since, to the subject.

Working with researchers in the UK, the US, Canada and Europe, Christopher and her team surveyed experts who treat postpartum runners and created guidance in 2024 on screening and steps to return to sports. Her newest research has also followed postpartum runners for more than a year, studying pelvic floor function, injury patterns, strength maintenance and biomechanical changes.

“We tried to pack a lot into this study, because there’s such a lack of longitudinal data to back most of the claims that are the current advice,” she said.

At six months into the study, her initial unpublished analysis shows variable results depending on the athlete. Overall, Christopher hypothesizes that the difference in outcomes varied because more active athletes stayed stronger than those who had to — or chose to — decrease their activity.

“We’re holding women back. If pregnancy and delivery was uncomplicated, they stayed active during pregnancy with minimal deconditioning, and their body’s ready to go back to sport, they should go,” Christopher said.

Real-life consequences of confusion

The lack of standardized, athlete-specific protocols leaves many in limbo, forced to choose between vague caution and their own intuition. And social media, Christopher warns, often fills that vacuum with anecdotes and opinions, amplifying confusion and sometimes fear. Without clear evidence, myths persist — and athletes hesitate.

Inadequate medical advice can lead to injury, burnout or premature retirement from sports, and the persistent myth that pregnancy is an athletic liability can discourage sponsorships and media coverage.

These consequences gained public attention in 2019, when Nike faced criticism for reducing compensation for pregnant athletes. Several prominent runners, including Allyson Felix and Alysia Montaño, shared their experiences of losing income and institutional support at the moment their bodies were undergoing some of their most extraordinary physical feats.

The backlash prompted policy changes at Nike: “In 2018 we standardized our approach across all sports to support all of our female athletes during pregnancy. While the specifics of each situation are unique, the policy waived performance reductions for 12 months. Additionally, the policy was expanded in 2019 to cover an additional 6 months, for a total of 18 months,” a Nike representative wrote in an email.

The episode underscored a broader issue: Athletic institutions often don’t recognize pregnancy as part of a performance arc. Instead, they treat it as an interruption.

Athletes like Huddle are actively working to change that, just as the researchers back them up with more data. In an Instagram post she shared on National Girls and Women in Sports Day, a few months before her second child was born, she wrote about how she hopes sports will evolve to fit all facets of the female athlete’s life.

“As a professional female athlete I always felt a lot of tension between my career and my ‘sports body’ and the idea of planning a family and my ‘woman body.’ The thing is they’re the same amazing body, but it felt like the expectation was to be one, then retire and be the other,” Huddle wrote. “It would have made me feel less stressed to have more information, resources, support and visible role models around all the ways you can thrive in both an athletic career and motherhood someday. I’d love the future of women’s sports to allow you to feel supported as your whole self the whole time.”

By Starre Vartan, CNN