What is more taxing on the human body—an ultra-marathon, or carrying a baby to term?

The answer is both, according to new research.

Scientists who claim to have found the ultimate level of human endurance say that pregnancy pushes the body to the same extremes as endurance events like long-distance triathalon competition Ironman or the Tour de France.

Researchers analyzed performance data from athletes competing in the Race Across the U.S.A.—a grueling 4,800 km, 120 day race from California to Washington, D.C.—and compared it to a variety of other endurance activities, such as long distance trekking and pregnancy.

When tracked over time, the data revealed that although athletes’ energy expenditure started out high, it eventually plunged and leveled out to burn calories at 2.5 times their resting metabolic rate.

The data suggests that the body can “downshift” its metabolism to stay within sustainable levels during these endurance events.

But researchers were surprised to find that the maximum energy expenditure among endurance athletes was only slightly higher than the metabolic rate women sustain during pregnancy, which peaks at 2.2 times higher than their resting rate.

Study co-author Herman Pontzer, an associate professor of evolutionary anthropology at Duke University, said the results all relate back to evolution.

“Human mothers have the biggest children and the longest pregnancies of all apes,” Pontzer told CTVNews.ca, noting that this could be a side effect of the human body evolving to achieve feats of endurance.

Does this mean that all human beings have the potential to conquer events like Ironman? Maybe, Pontzer says.

“I think we all have the potential—but how do we reach it? With pregnancy your body takes over and you have no control over it,” he explained.

“Every mother who has gone through a pregnancy has experienced that effort themselves.”

The research “defines the realm of what is possible for humans,” according to Pontzer.

Beyond the limit of 2.5 times a person’s resting metabolic rate, researchers say the body would start to break down its own tissue to make up for the lack of calories. The study suggests this could be due to the body’s inability to digest and absorb enough calories to sustain higher levels of activity.

“There’s just a limit to how many calories our guts can effectively absorb per day,” Pontzer explained in an article outlining the research.

The results of the study were published Wednesday in the journal Science Advances.