Scientists calculate the energy it takes to carry a baby—and it’s WAY more than they thought

Pregnant belly with stretch marks
Samantha Gehrman/Stocksy

You don’t need us to tell you that pregnancy is exhausting, from those early days when every symptom feels new to the aches and pains of the final weeks before delivery. A new study is shedding light on just how much energy it actually takes to carry a baby, and you’ll be entirely unsurprised to find out that scientists had previously underestimated it by a lot… a whole lot, in fact.

In the study, which was recently published in the journal Science, Australian researchers looked at the metabolic output required during a typical pregnancy, determining that the caloric demand equals around 50,000 dietary calories—the equivalent of about 50 pints of Ben and Jerry’s Cherry Garcia ice cream, which is apparently far more than the scientists had believed beforehand.

It seems that the previous estimates were lower because the widely accepted belief was that much of the energy required during reproduction was based in the uterus, but any person who’s ever been pregnant will tell you that pregnancy is a whole-body affair. In fact, 96% of the energy expended comes from the woman’s body, while 4% comes from the developing baby’s tissues.

Dustin Marshall, an evolutionary biologist at Monash University and one of the study’s authors, told the New York Times that he and his students scoured existing literature on the metabolic rates of dozens of species, including during reproduction, and the previous estimates didn’t quite seem to add up. The energy stored in a human baby’s tissues accounts for only about 4 percent of the total energy costs of pregnancy. The other 96 percent is extra fuel required by a woman’s own body.

While it might seem obvious, the researchers noted that the size of an animal has a direct impact on how much energy it needs to reproduce, while us large, warm-blooded mammals require even more energy to constantly feed the furnace. Of course, carrying a baby only adds to that caloric demand.

“It shocked me,” Marshall told the outlet. “We went back to the sources many times because it seemed astonishingly high based on the expectation from theory.”

“The baby itself becomes a rounding error,” he added. “It took us a while to wrap our heads around that.”

By aggregating such data, the researchers estimated the costs of reproduction for 81 species, from insects to snakes to goats.

Marshall’s research team found that the size of an animal directly influences how much energy it needs to reproduce. Microscopic animals, for example, may require less than a millionth of a calorie to make one offspring. On the other end of the spectrum, a white-tailed deer doe needs more than 112,000 calories to produce a fawn.

Marshall reports that there are myriad reasons why mammals pay such high indirect costs for being pregnant, and noted that many species build a placenta to transfer nutrients to their embryos. But humans likely pay a particularly high cost because women stay pregnant longer than most other mammals do.

Here’s hoping that the new findings will further cement what women have always known to be true: carrying a baby is a lot of work, and it takes plenty of fuel to keep you running during that time. All the more reason to take the time you need to rest, care for your body, and give it as much love as you can while you’re waiting for baby to arrive.