Women Lose Part Of Their Brain During Pregnancy And Scientists Think They Know Why

No, that's not a sensationalist headline. Women's brains really do shed an average of about 5% of their grey matter during pregnancy. The phenomenon is so universal, that it even birthed an informal term to describe it: "baby brain." Pregnant women with "baby brain" often experience forgetfulness, brain fog, disorientation, and problems focusing. Yet, while "baby brain" has long been recognized among scientists and expecting mothers as a real thing, it wasn't well understood until recently. Now, thanks to new insights provided by studies from organizations like the Bemother Research Initiative, scientists are finally uncovering the evolutionary mechanisms behind "baby brain."

But before expectant mothers start worrying, consider the facts. While it is true that there is a loss in brain matter during pregnancy, much of it is restored during postpartum. According to a 2025 study from Spain, published in Nature Communications, the brain's gray matter follows a "u-shaped trajectory" between the second trimester and six months after birth, meaning it dips in volume and then builds back up. This is actually a good thing, since it allows for heightened neuroplasticity, or the brain's ability to reorganize itself. Neuroplasticity is often reduced with age, so a mother's brain needs to make room for those motherly instincts to develop. One of the topic's lead researchers, Dr. Susana Carmona, compared the process to pruning a tree to allow for new growth.

While the loss of grey matter during pregnancy averages about 5%, the changes affect nearly the entire brain. Such changes are largely concentrated in the regions associated with social cognition. The researchers believe that this remodeling serves to help form a deeper connection between mothers and their babies. Tellingly, mothers who experienced greater changes in their brains were more likely to report a stronger bond with their babies.

Mothers in studies exploring "baby brain" typically show heightened levels of estrogen, the hormone now believed to be responsible for triggering the brain's restructuring during pregnancy. Indeed, hormones play a central role in ensuring that new parents care for their children. For instance, the unique "new baby smell" triggers a cocktail of hormones in the brains of new parents, hormones associated with love, social bonding, and protectiveness. And we're not the only animals that are chemically wired to care for our young — even cows experience an intense desire to keep their offspring close and the science behind "pessimistic cows" shows that they become depressed when they're separated from their calves.

Mice, too, experience vast hormonal and neurological changes during pregnancy. In fact, decades of murine studies, in which scientists study mice as models for humans, have shown just how important the "baby brain" changes are for survival. For instance, some research studies switched off the hormones in pregnant mice that trigger the brain to reorganize itself during pregnancy. The results were revealing: Without the brain's ability to reorganize itself in preparation for motherhood, mother mice largely ignored their babies. Their motherly instincts were virtually absent.

This makes sense evolutionarily, since most mammals are extremely vulnerable in their first few years of life and can't survive without parental protection. This is especially true for human beings. Because our ancestors were hunter-gatherers always on the move, protecting newborn babies from the dangers of the wild was critical for survival. Unsurprisingly, most of the changes in a pregnant mother's brain occur within the "default mode network," which is associated with empathy, altruism, and perception of self. Such qualities may seem like a survival impediment, but they're really the brain's way of ensuring survival for offspring. After all, self-sacrifice is the cornerstone of motherly love.