How does the stress hormone in the last months of pregnancy affect children's intelligence?

A new study shows that exposure to high levels of stress hormones during the last three months of pregnancy can reduce the IQ of male children before they are born.

Surprisingly, blood cortisol levels did not correlate with the girls' IQ scores, but higher levels in urine improved their scores.

The results highlight the important role that cortisol plays in independent fetal development in boys and girls.

When a woman is pregnant, her levels of cortisol naturally rise, which is a steroid hormone that is released in response to stress. It is essential for the healthy development of the baby and has a positive effect on his or her brain development.

However, researchers have found that excessive levels of the hormone during the last three months of pregnancy may hamper IQ scores in seven-year-old boys.

To examine its effect on children's cognitive function as they grow, researchers analyzed data on 943 pregnant women's cortisol levels during the last three months of pregnancy, and their children's IQ tests at age seven.

They discovered that pregnant women carrying a boy had lower levels of cortisol in the blood compared to women carrying a girl.

However, boys who were exposed to higher levels of cortisol in the womb scored lower on intelligence tests at age seven. While the girls don't seem to be affected.

The researchers, from the University of Odense in Denmark, said their findings indicate that boys may be “more exposed to cortisol before birth” compared to girls.

A previous study, conducted by the same research team, found that children between the ages of one and three years old had more advanced speech and language skills when their mothers had high levels of cortisol during the third trimester of pregnancy.

That study found that boys whose mothers were exposed to high levels of stress could say more words at the ages of 12 to 37 months, while girls were better at understanding more words at the ages of 12 to 21 months.

Lead author of the study, Dr. Anya Wenger-Dreyer, said: “Although our previous study showed that prenatal cortisol exposure was positively associated with language development, in this study prenatal cortisol exposure was negatively associated with IQ scores. This may mean that levels "High prenatal exposure may have a temporary effect on a child's cognitive development. It should also be noted that young children's vocabulary was self-reported in our previous study, while the child's IQ in this study was assessed by trained psychologists."

Separate research has also found that children exposed to high levels of cortisol are more likely to develop behavioral problems and stress-related illnesses later in life.

Meanwhile, experts have previously indicated that women who are stressed at the time of pregnancy are twice as likely to give birth to a girl.

The results were presented at the 26th European Endocrinology Congress in Stockholm, held May 11-14.