Analysis of 225 newborns reveals that prenatal hormone exposure, measured through finger length ratios, correlates with head circumference only in males. Boys with higher 2D:4D ratios—indicating greater first-trimester estrogen exposure—showed larger head circumferences at birth, while girls displayed no such relationship. This sex-specific pattern suggests prenatal hormones shape early brain development differently between males and females. The finding adds nuance to theories about human brain evolution, where some researchers propose that estrogenization during fetal development contributed to enlarged cranial capacity in our species. Previous work has linked 2D:4D ratios to various cognitive and behavioral traits, but this represents novel evidence for hormonal influences on newborn brain size markers. The male-specific correlation aligns with emerging understanding that sex hormones don't simply masculinize or feminize the brain uniformly, but create distinct developmental pathways. However, head circumference serves only as a proxy for brain development, and the study's cross-sectional design cannot establish whether these early differences persist or predict later cognitive outcomes. The research opens intriguing questions about whether hormonal influences on prenatal brain growth helped drive the evolutionary expansion of human intelligence.