Boys who develop autism are exposed to higher levels of steroid hormones in the womb than those who don't develop it, a study has concluded.
Prenatal levels of substances such as testosterone, progesterone and cortisol were greater on average in boys who were later diagnosed with autism, scientists at the University of Cambridge in England and the Statens Serum Institute in Copenhagen say.
The finding, published in the journal Molecular Psychiatry, provides a possible explanation for how autism develops during pregnancy, countering fears that external factors such as vaccines play a role.
Autism spectrum disorders, a group of brain development disorders, affect about one child in 160, according to the World Health Organisation.
"We previously knew that elevated prenatal testosterone is associated with slower social and language development, better attention to detail and more autistic traits," Simon Baron- Cohen, professor of developmental psychopathology at Cambridge, said. "Now, for the first time, we have also shown that these steroid hormones are elevated in children clinically diagnosed with autism."
The study drew on 19,500 amniotic-fluid samples stored in a Danish biobank from babies born between 1993 and 1999. The researchers identified samples from mothers who gave birth to 128 boys later diagnosed with an autism-spectrum condition. Because some of the hormones are produced in much higher quantities in males than in females, the finding may help explain why autism affects more boys than girls.
"We now want to test if the same finding is found in females with autism," Baron-Cohen said.
Mothers shouldn't rush to use steroid-hormone blockers, as this may have unwanted side effects, he said. The study also shouldn't be interpreted as indicating a need to develop a prenatal screening test, as the results were found at the average group level and may not predict diagnosis for an individual.
The study adds to earlier research suggesting that autism is linked to prenatal developments.
A paper published in March found that those diagnosed with autism missed key genetic markers for brain cells that are supposed to develop before birth.
A Norwegian study published last year found that taking folic acid supplements in early pregnancy was linked to a lower risk of autistic disorder. Folic acid is needed to fuse the spinal cord in early fetal development.