A first-of-its-kind study has revealed that the architecture of women’s brains changes strikingly during their first pregnancies, in ways that last for at least 2 years. In particular, gray matter shrinks in areas involved in processing and responding to social signals. This may mean that new mothers’ brains are more efficiently wired in areas that allow them, for instance, to respond to their infant’s needs or to detect threatening people in their environments. The changes correlated with standard tests of a mother’s attachment to her infant—and they occurred whether a woman conceived naturally or using in vitro fertilization.

“We certainly don’t want to put a message out there along the lines of ‘pregnancy makes you lose your brain,’” says the study's lead author Elseline Hoekzema, a neuroscientist at Leiden University the Netherlands who is also the pregnant mother of a 2-year-old. “Gray matter volume loss can also represent a beneficial process of maturation or specialization.”

Pregnancy is a time of dramatic, hormone-driven physiological and physical changes. Blood volume, hormone levels, absorption of nutrients, and other physiological capabilities grow dramatically. Other changes, according to anecdotal reports from pregnant women, are not so salubrious, like forgetfulness and difficulty concentrating. Whereas animal studies have shown that pregnancy is associated with apparently long-lasting anatomical brain changes—accompanied by adaptive changes, such as rodent mothers becoming better at foraging for food—virtually no studies have drilled down on anatomical changes in the human brain during pregnancy.

Hoekzema and her colleagues set out to change that. Working in Spain, in affiliation with the Autonomous University of Barcelona, they used MRI scanning to examine the brains of 25 women who had never had children, both before they became pregnant and again from 3 weeks to a few months after they gave birth. The team also scanned 19 first-time fathers at the same intervals, 17 men without children, and 20 women without children who did not become pregnant during the study. Then, they used computer-based analyses to measure changes in gray matter volume.

The findings showed highly consistent gray matter volume losses in the mothers and not in the other groups, the team reports today in Nature Neuroscience . The changes occurred primarily in areas of the brain involved in social tasks like reading the desires and intentions of others from their faces and actions. The hippocampus, a region associated with memory, also lost volume. What’s more, the team found that the mothers’ scores on a standard test that gauges the degree of a mom’s attachment to her infant could be predicted to a significant degree based on the changes in their gray matter volume during pregnancy.

The scientists also used MRI scans to watch the women’s brains work in real time, as they looked at photos of their own infants and of other babies. Several of the brain areas that had lost gray matter during pregnancy responded with the strongest neural activity to their own babies as opposed to the photos of other infants.(Comparisons between the brain’s response to photos of a mother’s own infant and to photos of other infants is a common measure researchers use to gauge neural responses to babies.)

Two years later, 11 of the 25 mothers—those who had not become pregnant again—returned for MRI scans. The scans showed that gray matter loss remained—except in the hippocampus, where most volume had been restored. The changes were so consistent that a computer algorithm could predict with 100% accuracy whether a woman had been pregnant from her MRI scan.

The researchers could not explain with certainty what the findings mean–they do not have the kind of access to the women’s brains that scientists have to rodents’, for instance—but they speculate that the gray matter losses might confer an adaptive advantage, Hoekzema says. She notes that a similar decline in gray matter volume occurs during adolescence, when neural networks are fine-tuned for more efficiency and more specialized functions.

Scientists not involved in the study noted that not only is it the first to demonstrate widespread anatomical changes in the pregnant human brain, but that it goes further by showing that the changes last for at least 2 years. “It opens the door to the possibility that it might cause changes in parenting that might have implications in decision-making and behavior later in life,” says Mel Rutherford, an evolutionary psychologist at McMaster University in Hamilton, Canada.

He adds that he would like to see similar studies in adoptive parents and in mothers who give up their children for adoption. That might strengthen the evidence from the current study that the changes arise solely from the physical fact of pregnancy and not, for instance, from the stress and sleep deprivation that all parents experience early in an infant’s life. (In the current study, the brains of the new fathers did not change despite these stresses.)

Abbe Macbeth, a neuroscientist with Noldus Information Technology, a behavioral research consultancy in Leesburg, Virginia, and herself the mother of 6- and 9-year-olds, says that less can be more when the brain restructures itself to respond to life changes. “There is all this anecdotal talk about pregnant women forgetting things, but that can occur in areas that don’t necessarily have anything to do with caring for our offspring,” she says. “That’s what nature wants us to focus on. This paper shows that.”