Research from Swansea University has revealed a connection between prenatal oestrogen exposure and head size at birth. The findings, drawn from two related studies published in the journal Early Human Development, point to subtle ways in which early hormonal environments shape human development from its earliest stages.
In one investigation, researchers examined the 2D:4D digit ratio in 225 newborns. This ratio, measured between the index and ring fingers, serves as a marker for prenatal hormone levels. A higher ratio signals greater oestrogen exposure relative to testosterone. Among the 100 boys and 125 girls studied, this elevated ratio was associated with larger head circumference in boys but showed no such link in girls.
Head circumference at birth tracks closely with brain size. Larger brains at this stage have long been tied to higher measures of intelligence later in life. The observation therefore carries implications for understanding variation in cognitive potential across individuals.
Direct measurement of maternal hormones
A second study looked at 47 mother-baby pairs. Scientists measured maternal oestrogen and testosterone concentrations in the first trimester. Levels taken at six to eight weeks of pregnancy positively predicted newborn head circumference. The relationship proved stronger in boys than in girls. By contrast, oestrogen levels at 10 to 11 weeks and testosterone concentrations at either point showed no predictive power.
Professor John Manning of Swansea University explained the evolutionary context.
This finding is relevant to human evolution because increases in brain size are found alongside feminisation of the skeleton, what is known as the oestrogenized ape hypothesis. High values of 2D:4D in males have been found to be related to elevated rates of heart problems, poor sperm counts and predisposition to schizophrenia. However, increases in brain size may offset these problems. Thus, the evolutionary drive for larger brains in humans may inevitably be linked to reductions in male viability including cardiovascular problems, infertility and rates of schizophrenia.
In remarks tied to the maternal hormone work, Manning added a further layer.