Growing a Baby Takes More Energy Than Running Marathons for 9 Months Straight

Of all the incredible feats the human body is capable of, from scaling mountains to completing ultramarathons, one of the most demanding endurance events is one that half the population can experience: pregnancy.

The popular notion that growing a baby is a monumental physical task is not just a feeling; it's a scientifically-backed reality.

Groundbreaking research has quantified the immense energy cost of gestation, revealing that carrying a child for nine months pushes the human body to the very edge of its sustainable limits, demanding more sustained energy than running a marathon every day for the entire duration.

The Science of Human Endurance and Running

To understand this incredible claim, one must first understand how scientists measure the body's energy use. The baseline for this measurement is the Basal Metabolic Rate (BMR), which is the number of calories your body burns to perform its most basic, life-sustaining functions while at rest.

Any activity, from walking to an intense tempo run, increases this rate.

For decades, scientists have studied the metabolic rates of elite runners to determine the ultimate ceiling of human endurance.

A landmark 2019 study led by researchers at Duke University provided a definitive answer.

The research team followed athletes participating in some of the world's most grueling endurance events, including the 2015 Race Across the USA, a staggering 3,000-mile run from California to Washington, D.C., that involved running six marathons a week for 20 weeks.

By tracking the athletes' energy expenditure, researchers discovered a clear pattern.

During extreme running events, an athlete's energy use would start high but would inevitably plateau at a sustainable maximum.

This ceiling was found to be approximately 2.5 times their Basal Metabolic Rate. The study concluded that exerting oneself beyond this 2.5x BMR limit is not sustainable over the long term.

The body simply cannot digest and absorb enough calories to fuel that level of output, forcing it to begin burning its own fat and muscle stores, which is a finite process.

This 2.5x BMR figure was identified as the hard, biological limit for sustained human performance.

Pregnancy: The Ultimate Endurance Run

The most astonishing finding of the Duke University study came when researchers compared the data from these elite ultrarunners to data on the metabolic demands of pregnancy.

They found that over the course of a nine-month pregnancy, a woman's body operates at an average of 2.2 times her resting metabolic rate every single day for about 270 days.

This places the sustained energy expenditure of pregnancy just shy of the absolute limit of human endurance identified in ultramarathon runners.

While a runner might hit a much higher metabolic peak during a single race a marathoner can reach 15.6 times their BMR for a few hours they cannot maintain it. The true feat of pregnancy is its duration.

It is the longest, most energetically expensive event the human body can sustain, far surpassing any single race.

Herman Pontzer, a Duke University professor and co-author of the study, emphasized this point, stating, "To think about pregnancy in the same terms that we think about...ultramarathon runners makes you realize how incredibly demanding pregnancy is on the body."

The research provides scientific validation for what mothers who run have instinctively known for millennia: growing a human life is the ultimate endurance event.

The Caloric Cost of Building a Human Runner-to-Be

So, exactly how much energy does this "40-week marathon" consume? A subsequent study published in the journal Science in May 2024 provided a startling number.

Researchers calculated the total energy cost of a full-term pregnancy to be approximately 49,753 dietary calories.

This figure is vastly higher than previous estimates. To put the 50,000-calorie cost into a runner's perspective:

• It is equivalent to the energy required to complete more than 30 full marathons, assuming an average burn rate of around 1,500-1,800 calories per marathon.
• It represents 96% of the energy the mother’s body expends to run the life-support system for the fetus, including building the placenta and increasing blood volume.

This enormous energy demand is precisely why the fatigue experienced during pregnancy can be so profound and persistent.

The body is not just building a baby; it is operating at a near-maximal metabolic state for nine consecutive months, a feat of endurance that rivals that of the most elite runners on the planet.

This research re-frames pregnancy not merely as a life stage, but as a demonstration of endurance that every running mom can be proud of.