Study: Differences between girls' and boys' brains emerge in utero

The human brain undergoes one of the most accelerated stages of development before and immediately after birth, but this process has been difficult to study as a continuous evolution. A recent study by researchers at the University of Cambridge, published in the journal Scientific Reports, shows that differences in brain development in girls and boys appear earlier than previously thought, being visible as early as the intrauterine period. Brain development is extremely intense in the second half of pregnancy and in the first weeks of life, but most previous research has focused either exclusively on the intrauterine period or on newborns and infants. This separation has left the transition between intrauterine and extrauterine life insufficiently explored. Without a continuous picture of this critical interval, it has been difficult to determine whether differences between male and female brains only emerge after birth or whether they start earlier and continue to increase thereafter.

• Tracking brain development as a continuous process

For the first time, a team of researchers from the Centre for Autism Research at the University of Cambridge has been able to track human brain development from mid-pregnancy to the first month after birth, using images obtained both prenatally and postnatally. The approach treated early brain development as a continuous process, rather than a sequence of discrete stages, allowing for a more precise interpretation of the rapid changes in volume and structure that occur during this period. The analysis was based on nearly 800 prenatal and postnatal brain scans, taken as part of the Developing Human Connectome Project, one of the largest imaging databases dedicated to the perinatal period. Large sample sizes are essential in studies of early development, as individual variation is considerable, and average differences between groups can be subtle and difficult to detect in small studies. The results, published in the journal Scientific Reports, show that there are prenatal differences in the rate of brain growth according to sex. On average, the male brain shows a more pronounced increase in total volume as development progresses, compared to the female brain.

The differences observed are not limited to certain regions, but are manifested throughout the brain, as early as the second half of pregnancy.

The study also provides a detailed picture of how different types of brain tissue contribute to the increase in brain volume during this critical period.

White matter, responsible for connecting different brain regions, has the greatest contribution to the increase in brain volume in mid-pregnancy. Towards the end of pregnancy and after birth, the dominant role is taken over by gray matter, involved in cognitive processes and information processing. At the same time, subcortical gray matter structures, such as the amygdala, cerebellum and thalamus, reach growth peaks earlier than cortical gray matter. This suggests that brain systems involved in basic functions mature before those associated with complex cognitive processes.

• Implications for understanding development and neurological diseases

The authors emphasize that establishing these trajectories of brain growth from the earliest stages of life is essential for understanding how early developmental differences can influence later development. Such differences could be relevant for explaining psychiatric and neurodevelopmental disorders, such as autism, which is associated with peculiarities in the rate of brain growth and organization.