When we think of human endurance, elite athletes running ultramarathons often come to mind. But new scientific research shows that pregnancy may be one of the most sustained physical challenges the human body can face.

A growing body of work suggests that there’s a long-term “metabolic ceiling” that humans simply can’t exceed. In a study published in Current Biology, researchers tracked ultra-endurance athletes, ultrarunners, cyclists, and triathletes, across weeks and months. They found that, on average, their daily energy expenditure plateaued at 2.5 times their basal metabolic rate (BMR), the rate at which energy is used when the body is at rest.

This limit appears to be deeply rooted in human biology. Even though athletes can briefly reach much higher energy outputs, over the long haul their bodies pull back, suggesting a natural cap on how much we can sustainably burn.

What does this have to do with pregnancy?

According to research led by anthropologist Herman Pontzer, the metabolic demands of pregnancy bring the body to around 2.2 times its resting metabolic rate, very close to that same biological limit. The difference is that, unlike a race that lasts days or weeks, pregnancy sustains this elevated energy output for approximately 270 days.

Pontzer and his colleagues argue that this prolonged energy drain makes gestation one of the most enduring metabolic feats a human body can sustain.

In 2024, a study published in science estimated that a full-term pregnancy demands roughly 50,000 dietary calories more than if the person weren’t pregnant.

What’s more, only about 4% of that energy goes into the developing foetus itself, the remaining 96% supports the growing metabolic demands of the mother’s body.

This finding challenges older, lower estimates and underscores just how much work the body is doing during pregnancy, not just building new tissue, but supporting every cell as it multiplies, powers up, and adapts.

Putting these studies together reframes pregnancy in a powerful way. It’s not simply a natural process: from an energy perspective, it’s a long-duration endurance event, one that demands more than many elite athletes sustain during their most gruelling competitions.