Brain scientists have long debated when, and how strongly, sex starts to shape the human brain. A growing body of imaging work now points to a clear answer: measurable differences in brain size and structure appear before a baby takes its first breath. Those findings are forcing a rethink of what is “built in” during pregnancy and what emerges later through experience.
At the same time, researchers are at pains to stress that early-average differences in brain volume do not translate into fixed limits on what any individual can think, feel, or achieve. Instead, the new data sharpen questions about vulnerability to conditions such as autism and ADHD, and about how prenatal biology interacts with the social worlds children are born into.
Inside the new womb‑to‑newborn brain maps
The most striking recent work tracks brain growth continuously from the middle of pregnancy into the first weeks of life, rather than relying on isolated snapshots. In one project, Researchers followed fetuses from mid‑pregnancy through the first month after birth, using repeated scans to build growth curves for each baby. That approach, developed at the Autism Research Centre at the University of Cambridge, allowed the team to see how brain volume changed over time in the same child, rather than inferring development from different infants scanned once.
Those longitudinal curves revealed that sex-linked differences in total brain volume were already visible in the womb and continued to widen across the perinatal period. A related Abstract describing this mapping work characterizes the prenatal and early postnatal window as a “highly dynamic and foundational phase” in which male brain volume tends to increase more steeply with age compared with females. In other words, the divergence is not a late tweak, it is baked into the earliest growth trajectories.
How big are the differences at birth?
To understand what those trajectories look like at the moment of birth, I look to large MRI datasets of healthy newborns. One analysis of 514 newborns, including 236 birth‑assigned females, reports that, on average, male infants have significantly larger global brain volumes. A companion Abstract on the same work notes that these sex differences in human brain anatomy are already present at birth, even though the field has historically focused on adults.
Other teams have converged on similar patterns. A project summarized as Study Confirms Early examined over 500 newborns and again found that male infants tend to have larger total brain volumes. A separate Background note on that work emphasizes that these are on‑average differences, with substantial overlap between individual boys and girls.
Grey matter, white matter, and what “bigger” really means
Size is only part of the story. When researchers adjust for overall brain volume, they see a more nuanced picture of how tissue types differ between the sexes. A review on infant BRAIN MORPHOLOGY notes that one of the most consistent findings, labeled section 3.1, is that male brains are larger in absolute volume in both infancy and adolescence. Yet when scientists correct for that size difference, female infants often show proportionally more grey matter, while males show relatively more white matter.
That pattern is echoed in a neonatal imaging Study reporting that Sex differences in brain structure are present from birth, with male infants tending to have more white matter and female infants, when adjusted for brain size, having more grey matter. A discussion thread summarizing the same work notes that while male infants tend to have larger total brain volumes, female infants have more grey matter and male infants more white matter, a point that is captured in a Mar summary. Supporting that, a separate report explains that White matter consists of long nerve fibres connecting distant brain regions, hinting at why even subtle shifts in its volume might matter for information flow.
Hormones, trajectories, and the prenatal origins of difference
What could be driving these early divergences in brain volume and tissue composition? One line of evidence points to sex steroid hormones that surge during fetal life. A detailed review Abstract notes that Most of the anatomical, physiological and neurochemical gender‑related differences in the brain occur prenatally, in a period when Male fetuses are exposed to much higher levels of hormones such as testosterone. A separate report on how Male and female brains start developing differently in the womb underscores that these hormones are known to shape sex differences in the brain in animals, and the new human data suggest the same is true in people.
Those hormonal influences are not just theoretical. A study of prenatal hormone exposure and later behavior reports that Associations between hormones and composite behavioral scores were evaluated using multiple linear regressions in both sexes combined, and then separately, highlighting how early biology can leave measurable traces in childhood. Another imaging analysis of fetal and infant scans, summarized under the prompt “What is this?”, found that male brain volume followed a different growth curve than female brain volume, with males showing a more rapid early expansion while females showed a more linear trajectory. A separate report on Sex differences in brain growth emerging in the womb, led by Cambridge researchers, explicitly links those growth patterns to prenatal exposure to testosterone and estrogen.
From first breath to early development: what changes, what stays stable
Once babies are born, the question becomes whether these early-average differences fade, intensify, or simply shift in form. A detailed Paper Summary titled Methodology Explained describes how scientists used MRI to scan the brains of healthy, full‑term newborns and then followed them over time. That work, echoed in a separate summary titled Methodology, suggests that sex differences in brain structure are present at birth and remain relatively stable during early development, rather than flipping or disappearing in the first months.
Online discussions of this work, including a widely shared Sex differences summary, emphasize that while male infants tend to have larger total brain volumes, female infants, when adjusted for brain size, have more grey matter, whereas male infants have more white matter, and that these patterns remain stable during early development. A separate report on Study sheds light on early brain differences between genders notes that these structural patterns may help explain why some neurodevelopmental conditions are diagnosed more often in males, a point also highlighted in the Study Confirms Early summary.
More from Morning Overview
*This article was researched with the help of AI, with human editors creating the final content.
