Pregnancy puts a significant strain on the body, requiring an increased intake of food to support both the pregnancy and the growing baby. But just how many extra calories are needed to grow a baby? Estimates vary widely, ranging from roughly 50,000 to nearly 85,000 additional calories throughout the entire pregnancy. These figures represent calories on top of what a woman would require if she wasn’t pregnant.
According to , a professor of evolutionary anthropology and global health at Duke University, “For most women, 50,000 calories is going to be a gross underestimate. I think, for most, it’s going to be more like 70,000… or even more.” Scientists have reached these estimations through various methods. For example, a 2024 study published in the journal developed a formula to calculate the calorific cost of pregnancy for numerous species across the animal kingdom. Led by , a doctoral student at Monash University at the time of the study, the researchers analyzed 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 explained in an email to Live Science. However, only 4% of those 50,000 extra calories directly contribute to the fetus’s cellular growth. The majority are used to support the pregnant person’s body as it undergoes transformations throughout pregnancy. This is because, as Pontzer, who was not involved in the study, pointed out, “Fundamentally, energy expenditure is all about all of your cells doing their jobs all day.” During pregnancy, the body grows approximately 12 kilos (26 pounds) in a typical pregnancy, creating extra tissue and cells that require energy to function.
The energy requirements of pregnancy vary throughout the nine months. According to a 2005 paper in the journal , the first trimester demands the least amount of extra energy. For an average, healthy pregnant woman, the weight gain during this time is around . This increases to 2.1 ounces (60 grams) per day during the second trimester and then slightly decreases to 1.9 ounces (54 grams) per day in the final trimester. However, Pontzer believes that the calories needed to sustain this extra tissue and grow a new human are likely higher than the study by Ginther and colleagues suggests.
“The real advancement in this new paper is that they’ve looked so broadly across the tree of life” — encompassing reptiles, amphibians, fish, and mammals, Pontzer noted. “But we’ve known for a long time — thanks to groundbreaking work by — 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 published a study that calculated the total energy cost of pregnancy, reaching a figure of 77,675 calories. This equates to 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.
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A review by a different group, published in 2019 in the journal , examined the work of Butte and others, concluding that 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. When added up, this amounts to roughly 78,400 to 84,700 additional calories over the nine months.
Why are there discrepancies in the estimations of calories needed to grow a baby? “To give one number is going to be tough,” Pontzer explained. “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.” Additionally, the amount of energy needed also depends on factors such as a person’s physical activity level and other physiological traits, such as their metabolism. Nonetheless, Pontzer emphasizes that, based on previous research, most pregnancies likely require well over 70,000 extra calories to maintain.
