Pregnancy places a lot of extra strain on the body, requiring more food than usual over the course of nine months to support the pregnancy and the growing baby.
But exactly how many calories does it take to grow a baby?
It turns out that estimates range widely — from about 50,000 to nearly 85,000 extra calories over the course of an entire pregnancy. Those are additional calories on top of what that person would need if they weren't pregnant.
"I would say that, for most women, 50,000 calories is going to be a gross underestimate," said Herman Pontzer, a professor of evolutionary anthropology and global health at Duke University. "I think, for most, it's going to be more like 70,000 … or even more."
Related: How calories are calculated: The science behind your food
Scientists have arrived at these numbers using different methods. For instance, a 2024 study published in the journal Science devised a formula for calculating the calorific cost of pregnancy for many species across the animal kingdom.
The team was led by Samuel Ginther, who was a doctoral student at Monash University at the time of the study. The researchers worked out the reproductive cost of 81 species, ranging from microscopic, aquatic animals to large mammals, including humans.
"We calculated that a pregnant person would require an additional 50,000 kcal [calories] over a 9-month period compared to a similar non-pregnant female over the same time period," Ginther told Live Science via email.
Of the 50,000 extra calories needed in pregnancy, the team estimated that just 4% go directly into growing the cells of the fetus. The majority are instead used to support the pregnant person's body as it changes throughout pregnancy, said Pontzer, who was not involved in the study.
"Fundamentally, energy expenditure is all about all of your cells doing their jobs all day," Pontzer said. "In pregnancy — when the body grows something around 12 kilos [26 pounds] in a normal pregnancy — all that extra tissue [is] all extra cells that weren't there before. And they've all got to do their jobs."
The energy demands of pregnancy change over the course of the nine months. The first trimester takes the least amount of added energy, according to a 2005 paper in the journal Public Health Nutrition. During this time, the weight gain for an average, healthy pregnant woman is around 0.6 ounce (18 grams) per day. This increases to 2.1 ounces (60 grams) per day during the second trimester and then decreases slightly to 1.9 ounces per day (54 grams) in the final trimester.
Related: Having a baby: Stages of pregnancy by trimester
The calories needed to support this extra tissue and grow a whole new human are likely higher than Ginther and colleagues' study suggests, Pontzer said.
"The real advancement in this new paper is that they've looked so broadly across the tree of life" — at reptiles, amphibians, fish and mammals, he said. "But we've known for a long time — thanks to groundbreaking work by Nancy Butte — that the energy costs of [human] pregnancy are upwards of 70,000 calories."
Butte, a professor of pediatrics and nutrition at Baylor College of Medicine, and her colleagues once published a meta-analysis of several studies that calculated the total energy cost of pregnancy. The figure came out to 77,675 calories. This would consist of an extra 90 calories per day in the first trimester, 287 calories per day in the second trimester and 466 calories per day in the third.
A review by a different group, published in 2019 in the journal Nutrients, looked over Butte's and others' work and concluded the needs of pregnancy ranged from 50 to 150 extra calories per day in the first trimester, 340 calories per day in the second and 452 calories per day in the third. Added up, that amounts to about 78,400 to 84,700 additional calories across the nine months.
So, why are there discrepancies as to how many calories it takes to grow a baby?
"To give one number is going to be tough," Pontzer said. "A small woman is going to have probably a different energy cost than a big woman, just because we know that the energy cost of everything scales with size." Plus, the amount of energy needed also depends on how physically active a person is and other physiological traits, such as their metabolism.
But again, at least based on past research, Pontzer said, most pregnancies probably require well over 70,000 extra calories to sustain.
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