A new study has examined just how mothers influence the size of their child’s head – and as a result, its brain size and future IQ – during the first trimester of their pregnancy.
The evidence for this rests on indirect studies of prenatal oestrogen as measured by digit ratio - the relationship between the length of index and ring fingers, known as the 2D:4D ratio. Newborns who have experienced high oestrogen relative to testosterone have long index fingers (2D) relative to their ring fingers (4D) and large head circumference.
Now a collaboration between Swansea University and the Medical University of Lodz has put this theory to the test by relating concentrations of early sex hormones in the mother’s blood to their newborn’s head circumference.
Their findings have been published in journal Early Human Development.
Digit ratio expert Professor John Manning , of Swansea’s Applied Sports, Technology, Exercise and Medicine (A-STEM) research team, said the study shows prenatal oestrogen is positively related to neonate head circumference.
In a sample of 47 mother-baby pairs the team found that maternal oestrogen levels at six to eight weeks of the pregnancy predicted the baby’s head size at birth and that this influence was stronger for boys than for girls.
The researchers say this is important for our understanding of human evolution because increases in brain size are found alongside oestrogenization of the body - the so-called oestrogenised ape hypothesis which seeks to explain how early humans developed such large brains.
Professor Manning said: “High values of prenatal oestrogen in male babies are related to subsequent heart problems and low sperm counts. Increases in brain size may offset these fitness problems. Thus, the evolutionary drive for larger brains in humans may inevitably be linked to reductions in male heath and fertility.”
Professor Manning’s previous research has examined how digit ratio may provide valuable information relating to alcohol consumption, outcomes after contracting Covid-19, as well as oxygen consumption in footballers.
Early Human Development
10.1016/j.earlhumdev.2026.106604
Experimental study
People
First trimester maternal sex steroids and head circumference in newborns
12-Jun-2026
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