Medical University of Lodz

Prenatal Estrogen in Early Pregnancy Linked to Male Newborn Brain Size, Twin Studies Show

- Two convergent studies published in Early Human Development link first-trimester prenatal estrogen exposure to larger head circumference in male newborns. - A digit-ratio study of 225 newborns found higher 2D:4D ratios in boys associated with larger head circumference; a direct blood-chemistry study of 47 pregnancies confirmed the same pattern. - The findings provide the most direct empirical support yet for the estrogenized ape hypothesis, which proposes that rising prenatal estrogen drove both skeletal feminization and brain expansion in human evolution. - Researchers caution that head circumference is a proxy for brain volume, not intelligence, and that the evolutionary trade-off may carry male-specific health costs including cardiovascular problems and reduced fertility.