HMN 2026: How Sex differences in brain growth emerge in the womb

Cambridge researchers have revealed a detailed picture of how the human brain grows from mid-pregnancy through the first weeks after birth and identified that sex differences in brain growth are apparent from mid-pregnancy onward. Their study is published in the journal Scientific Reports.

Filling a key research gap

There has long been debate over exactly how early in human brain development sex differences first emerge, and what causes them. Previous research has typically studied prenatal or postnatal brain development alone but not both together. That means that until now it has not been possible to study brain growth across the prenatal to postnatal transition.

Researchers at the Autism Research Center, University of Cambridge, analyzed data that mapped how the human brain develops continuously across the prenatal to postnatal transition. This enabled them to develop a more accurate model of early human brain development and to pinpoint exactly when sex differences in brain growth first emerge.

The study used one of the largest perinatal brain imaging datasets ever assembled, collected by the Developing Human Connectome Project. The dataset consisted of nearly 800 prenatal and postnatal brain scans from mid-pregnancy to one month post-birth.

Early sex differences in growth

Yumnah Khan, a Ph.D. student in the Autism Research Center, who led the study, said, “The human brain undergoes its most rapid and complex development before and shortly after birth. But until now, very little was known about exactly how the brain grows during this formative period of life, and how males and females might differ in this process. Our study has documented the presence of prenatal sex differences in the growth of the human brain.”

The team identified that, on average, males showed greater increases in brain volumes with age, across the whole brain, compared to females.

Possible hormonal influences

Dr. Alex Tsompanidis, a Senior Research Associate at the Autism Research Center, and a member of the research team, said, “This study addresses the age-old question of whether nature plays a role in shaping sex differences in the brain. The findings suggest that prenatal biology sets the stage for such sex differences, even if postnatal experience influences these further.

“The next step is to test if the observed sex differences in human brain growth are driven by prenatal sex steroid hormones, such as testosterone and estrogen. Male fetuses are exposed to much higher levels of these hormones, which we know play a role in shaping sex differences in the brain and behavior in other animals. We need to test if the same is true in humans.”

How different brain tissues mature

The research also provided several other important insights into how the brain grows during early development. For example, different brain regions and tissues were found to mature at different rates. White matter—responsible for connecting different brain regions—was found to be the main contributor to brain growth during mid-pregnancy, while gray matter—responsible for cognition and information processing—was found to dominate growth during late pregnancy and after birth.

The researchers also found that early brain development is carefully timed to meet ongoing developmental demands. For instance, subcortical gray matter structures (those deeper within the brain, such as the amygdala, cerebellum, and thalamus) show earlier peak growth rates than cortical gray matter, suggesting that brain systems supporting basic functions mature earlier than those involved in higher-order cognition.

Links to autism and neurodivergence

Dr. Richard Bethlehem, an Assistant Professor in Neuroinformatics, and a member of the team, said, “Establishing these brain growth trajectories early in life is critical because these may help us understand how differences in early brain development contribute to diverse outcomes, including psychiatric and neurodevelopmental conditions such as autism, which is associated with differences in rates of brain growth.”

Professor Sir Simon Baron-Cohen, Director of the Autism Research Center, who supervised the study, added, “These findings may help us understand why males and females show differences in the likelihood of neurodivergent outcomes such as autism. For example, the early sex differences in the brain may be due to prenatal sex steroids, and autistic people are exposed to elevated levels of prenatal sex steroid hormones. Future research needs to join the dots in this exciting field of developmental neuroscience.”

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