Pregnancy and the Brain: Is “Momnesia” Real?
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I have three adult children, so I’m part of the nearly 85 percent of women worldwide who experience at least one pregnancy during their lifetimes. Becoming a mother is a true rite of passage that changes us in significant and often challenging ways, both physically and psychologically.
The sudden sense of 24/7 responsibility can be daunting. Sleep deprivation takes on a whole new meaning. We require the skills of a logistical engineer to venture out of the house. It’s hard to feel attractive when you have peanut butter and jelly in your hair. Any negatives, however, are easily offset by the awe we experience while watching the development of a completely new, unique person.
One particular change that is a staple of mommy discussions is “momnesia,” or “pregnancy brain.” Pregnant women often complain about sudden lapses in memory. We all have had the experience of walking into a room and then completely forgetting what we planned to do, but pregnant women seem to report these lapses more frequently than others of the same age. Are these experiences due to tiredness and sleep deprivation? Or are there other explanations?
Neuroscientists have begun to ask to what extent these anecdotal observations are associated with real physical changes in the brains of pregnant women and new mothers. This is not at all far-fetched. We know that sex hormones impact the brain and behavior in significant ways (think puberty), and pregnancy unleashes a literal tsunami of hormones. Those hormones interact with receptors in the nervous system as well as in the reproductive system, leading to structural and functional changes.
How Does the Brain Change During Pregnancy?
Investigations in animals clearly show that pregnancy changes the brain and does so in ways consistent with improved maternal care, at least in those species where the mother sticks around long enough to provide care. From an evolutionary standpoint, this makes perfect sense. Anything that promotes improved parental care is likely to enhance the survival of the offspring.
Human beings are especially in need of care. We stand out in the animal kingdom for our incredible helplessness and dependency at birth. We have to be between 12 and 18 months old before we can do what other primates can at birth. Our dependency lasts a long time, at least 18 years in developed societies. It makes sense, then, that we would experience changes during pregnancy that promote nurture.
What types of brain changes support improved maternal care? It’s often much easier for scientists to investigate questions like this using laboratory animals. They reproduce quickly, their environment can be controlled entirely, and there are few ethical minefields as intense as research on pregnant women.
You might not think of rodents as being role models for motherhood. Still, mother rodents engage in various maternal behaviors, including nest-building, pup retrieval, and licking or grooming behavior (the rodent equivalent of a hug from your mom). The hypothalamus, a central hub for hormonal and regulatory functions, is intimately involved in these behaviors. As pregnancy-related hormones increasingly influence the rodent hypothalamus, structural changes occur, leading to behavioral changes. Females that normally avoid pups now approach them. Mother rodents become more sensitive to sensory cues from newborn pups. Conversely, inhibiting these hormones reduces these maternal behaviors. Treated females are less likely to engage in nest-building or pup retrieval.
Do we see analogous changes in human brains? The answer is yes. Klara Spalek and her colleagues (2024) reported that comparisons between 107 mothers and 57 women who had never been pregnant showed that pregnancy was linked to structural changes in the hypothalamus. These changes were correlated with hormonal levels during pregnancy as well as with self-reported late-pregnancy behaviors, such as maternal attachment and nesting behaviors.
Memory is Complicated, But Do We See Changes in Higher Levels of the Brain?
The basic regulatory actions performed by the hypothalamus are important, but they run in the background like your antivirus program. Changes in complex behaviors like memory imply structural modification in higher brain levels, especially the cerebral cortex. Do we see evidence of pregnancy-related changes there?
Several investigators doing before-and-after comparisons have noted that gray matter volume in the cerebral cortex decreases during pregnancy. These are not short-lived changes and might be permanent. Reductions in gray matter volume might sound scary. Am I losing computing capacity?
In this case, however, viewing the reduction as a way to streamline processing is probably more accurate. A similar drop in cortical thickness occurs during adolescence, as unnecessary connections are pruned to produce a mature adult brain. You can think of the brain as analogous to a sculptor’s block of marble. As the sculptor chips away, the beautiful final product emerges.
The association cortex is the area of the cerebral cortex most likely to experience a decrease in volume during pregnancy. This area integrates sensory data, possible responses, and abstract thinking to support our highest cognitive functions. In particular, areas associated with the processing of theory of mind (ToM) experience notable change. Theory of mind encompasses our human abilities to empathize or put ourselves in another person’s shoes. It’s hard to imagine a more necessary skill for parents, especially when navigating the needs of nonverbal infants.
While many studies have used a before-and-after approach, Laura Pritschet and her colleagues (2024) tried something novel. They followed the brain changes of one individual woman from three weeks before conception to two years after she gave birth using MRI scanning and hormone assessments. As their participant’s pregnancy progressed, the researchers observed a significant reduction in cortical gray matter volume that corresponded to increases in sex hormones. As in previous observations, the woman experienced hypothalamus and other subcortical areas changes. Most relevant to our question of memory, gray matter volume was also reduced in parts of the hippocampus, a structure known to play important roles in memory formation and retrieval.
At the same time, the researchers observed gradually improving white matter integrity as the pregnancy progressed, indicating that connectivity between structures was being strengthened. Many of these changes persisted to the two-year postpartum assessment.
Implications and Next Steps
Observations of brain changes during and after pregnancy have the potential to significantly contribute to our understanding of women’s health, parenting behaviors, and counseling approaches. The methods used by Pritschet and her colleagues can help pinpoint underlying correlates of individual differences in parental behavior, mental health, and brain aging. An obvious next step would be to replicate these findings across a large and diverse sample of women.
But what about our initial question of momnesia or pregnancy brain? Should women worry about this? Probably not. Any changes that occur are likely happening for some very good reasons. Structural and functional changes in the brain clearly accompany pregnancy, but these changes do not act in isolation.
Pregnancy is a profound period of social change, affecting many of our relationships. Sleep difficulties, stress, and increased cognitive load could very well contribute to the experience of pregnancy brain. As any introductory psychology student can tell you, we are not always the best observers of our own behavior. Through further controlled studies, however, we will gain new insights into the effects of pregnancy on brain structure and function and the subsequent behaviors that result from these changes.
Laura Freberg, PhD
Writer & Contributing ExpertLaura Freberg serves as professor of psychology at Cal Poly, San Luis Obispo, where she teaches introductory psychology and behavioral neuroscience.
Dr. Freberg is the author or co-author of several textbooks, including Discovering Psychology: The Science of Mind, Discovering Behavioral Neuroscience, Applied Behavioral Neuroscience, and Research Methods in Psychological Science. She served as President of the Western Psychological Association (WPA) in 2018-2019.