If you've ever known a pregnant woman, you'll be well aware of the fact that she's a superhero.

But now it has been officially confirmed how incredible women carrying babies are, with a new study claiming they are close to living "at the limit of what the human body can cope with".

In the study, published last week in Science Advances, researchers from Duke University analysed a 3,000-mile run, the Tour de France and other elite events.

They found that the cap of physical endurance was 2.5 times the body's resting metabolic rate - the calories the body burns through when it is relaxing - or 4,000 calories a day for an average person. Beyond this, human bodies are unable to digest, absorb and process enough calories in the long term.

The maximum energy expenditure among endurance athletes was only slightly higher than metabolic rates women sustain during pregnancy, with a woman's energy use peaking at 2.2 times their resting metabolic rate in the nine months.

To measure the rate, the scientists studied runners who were taking part in the Race Across America. The participants ran 3,080 miles between California and Washington over the course of 140 days, the same distance as six marathons a week.

Over the period of time, scientists studied the effects of the physical efforts on their bodies, with both resting metabolic rate and calories burned in the extreme endurance event recorded before and during the race.

The study found a pattern between the length of a sporting event and energy expenditure - the longer the race went on, the tougher it was to burn through the calories.

This means people can go far beyond their base metabolic rate while doing a short period of exercise, however it becomes unsustainable in the long term.

"You can do really intense stuff for a couple of days, but if you want to last longer then you have to dial it back," Dr Herman Pontzer, from Duke University, told BBC News. "Every data point, for every event, is all mapped onto this beautifully crisp barrier of human endurance. Nobody we know of has ever pushed through it."

If you've ever known a pregnant woman, you'll be well aware of the fact that she's a superhero.

But now it has been officially confirmed how incredible women carrying babies are, with a new study claiming they are close to living "at the limit of what the human body can cope with".

In the study, published last week in Science Advances, researchers from Duke University analysed a 3,000-mile run, the Tour de France and other elite events.

They found that the cap of physical endurance was 2.5 times the body's resting metabolic rate - the calories the body burns through when it is relaxing - or 4,000 calories a day for an average person. Beyond this, human bodies are unable to digest, absorb and process enough calories in the long term.

The maximum energy expenditure among endurance athletes was only slightly higher than metabolic rates women sustain during pregnancy, with a woman's energy use peaking at 2.2 times their resting metabolic rate in the nine months.

To measure the rate, the scientists studied runners who were taking part in the Race Across America. The participants ran 3,080 miles between California and Washington over the course of 140 days, the same distance as six marathons a week.

Over the period of time, scientists studied the effects of the physical efforts on their bodies, with both resting metabolic rate and calories burned in the extreme endurance event recorded before and during the race.

The study found a pattern between the length of a sporting event and energy expenditure - the longer the race went on, the tougher it was to burn through the calories.

This means people can go far beyond their base metabolic rate while doing a short period of exercise, however it becomes unsustainable in the long term.

"You can do really intense stuff for a couple of days, but if you want to last longer then you have to dial it back," Dr Herman Pontzer, from Duke University, told BBC News. "Every data point, for every event, is all mapped onto this beautifully crisp barrier of human endurance. Nobody we know of has ever pushed through it."