If you've ever thought pregnancy was a lot like running a nine-month marathon then you're not alone - it's exhausting, gruelling, sweaty and takes a hell of a lot of endurance. Well, according to science, it actually kind of is.
A new study has shown that just like completing the Tour de France or doing a triathlon, pregnancy pushes the body to its maximum limits, too.
"To think about pregnancy in the same terms that we think about Tour de France cyclists and triathletes makes you realise how incredibly demanding pregnancy is on the body," co-author Herman Pontzer, an anthropologist at Duke University, North Carolina told Science.
In research published in Science Advances, Pontzer and a team from Duke University measured the amount of calories burned each day by a group of athletes who ran six marathons a week for five months as part of the 2015 Race Across the USA. (Yep, just a cheeky six a week). They then compared their energy expenditure rate with athletes from other endurance events (triathlons, Tour de France, and arctic trekking) as well as in pregnant women.
The researchers found that when it comes to physical activities lasting days, weeks and months, humans can only burn calories at 2.5 times their resting metabolic rate, or an average of 4,000 calories per day. Beyond this magic number, the body starts breaking down its own tissues.
But here's where it gets interesting - the maximum sustainable energy expenditure found among endurance athletes was only slightly higher than the metabolic rates women sustain during pregnancy, which is 2.2. Studies show that we tend to burn twice as many calories during pregnancy during which, as the authors, note, "weight gain is imperative".
According to the researchers the same physiological limits that athletes face when trying to break records, may also have an impact on other aspects of life such as how big babies can grow in the womb.
"This defines the realm of what's possible for humans," said Pontzer of the findings.
So there you go. It's not just your imagination - pregnancy is all about endurance.
We give you permission to go and take a nap.