Stress during the later stages of pregnancy can lower the intelligence of boys before they’re even born, warns a new study.
Higher levels of the stress hormone cortisol during the third trimester may impede the IQ scores of boys at seven years of age, according to the findings.
Cortisol levels in the blood were not associated with IQ scores in girls — but higher urine cortisone levels actually improved their scores, say scientists.
They say their findings highlight the important role cortisol plays in foetal development in boys and girls independently.
The Danish research team explained that prenatal exposure to cortisol — a steroid hormone that helps the body respond to stress — is needed for foetal development.
It is thought to affect brain function in children later in life.
During pregnancy, the levels of cortisol increase and pregnant women carrying girls usually secrete more cortisol than those with boys.
However, in the placenta, an enzyme called 11beta-HSD2 regulates the amount of cortisol that reaches the foetus by converting cortisol to its inactive form, known as cortisone.
The Danish team had previously shown that children between the ages of one- and three-years-old have more advanced speech and language skills when their mums had high levels of cortisol during their third trimester.
For the new study, the researchers analysed data on the cortisol and cortisone levels of 943 pregnant women during the third trimester and on the IQ tests of their 943 children, aged seven.
They found that pregnant women carrying a boy had lower cortisol levels circulating in their blood than those women carrying a girl.
And boys exposed to higher cortisol levels in the womb scored lower on IQ tests at age seven.
But girls the same age scored better on IQ tests when their mothers had higher levels of urine cortisone.
Study lead author Dr Anja Fenger Dreyer said: “To our knowledge, this is the first study investigating the association between urine cortisone levels during pregnancy and IQ scores in children.
“While other studies have only looked at cortisol circulating in the blood during pregnancy and child IQ, we are the first to look at urine samples as well as blood samples and to investigate boys and girls separately.
“Our results show that girls may be more protected by the activity of placental 11beta-HSD2, whereas boys may be more vulnerable to prenatal exposure of maternal physiological cortisol.
“Although our previous study showed prenatal cortisol exposure was positively associated with language development, in this study prenatal cortisol exposure – ‘directly’ by serum cortisol and ‘indirectly’ by urine cortisone – is negatively associated with IQ scores.
“This may mean that the high levels of prenatal cortisol exposure might have a temporary effect on a child’s cognitive development.”
Dr Fenger Dreyer, of Odense University Hospital, added: “It should also be noted that the vocabulary in toddlers was reported by parents in our previous study, while child IQ in this study was assessed by trained psychologists.”
The findings are due to be presented at the 26th annual European Congress of Endocrinology in Stockholm, Sweden.
