Scientists believe oestrogen affects brain growth in foetuses, but it is not currently clear whether these elevated hormone levels come from the mother, baby or placenta.

The National Autistic Society defines autism as a lifelong, developmental disability that affects how a person communicates and relates to other people and how they experience the world around them.

It is not a disease but rather a spectrum disorder. Due to the nature of this disorder, people with autism all have different experiences.

As part of the new study, scientists tested the amniotic fluid from 98 people sampled from the Danish Biobank which has amniotic samples from more than 100,000 pregnancies. All four oestrogens were significantly higher in foetuses who later developed autism, compared to 177 foetuses who did not.

It was previously thought that testosterone was linked to autism, but scientists say that oestrogen has an even stronger link.

“This new finding supports the idea that increased prenatal sex steroid hormones are one of the potential causes for the condition,” said lead researcher Professor Simon Baron-Cohen, Director of the Autism Research Centre at the University of Cambridge.

“Genetics is well established as another, and these hormones likely interact with genetic factors to affect the developing foetal brain.”

Scientists say the find is particularly exciting because the role of oestrogen in autism has hardly been studied, according to the paper published in Molecular Psychiatry.

What is autism spectrum disorder?

Dr Alexa Pohl, part of the Cambridge team, said: “We hope that we can learn more about how they contribute to foetal brain development in further experiments. We still need to see whether the same result holds true in autistic females.”

It is the latest theory to support the prenatal sex steroid theory of autism which was first proposed 20 years ago.

It follows 2015 research in which scientists from the University of Cambridge and the State Serum Institute in Denmark measured the levels of four prenatal steriod hormones in the amniotic fluid in the womb.

Researchers found that two of these hormones, known as androgens, were higher in male foetuses who went on to develop autism. These androgens are produced in higher quantities in male than female foetuses which could explain why autism is seen more often in boys.

Professor Baron Cohen warned that the findings cannot and should not be used to screen for autism. “We are interested in understanding autism, not preventing it,” said Professor Cohen.

Dr Arieh Cohen, the biochemist on the team, based at the State Serum Institute in Copenhagen, said: “This is a terrific example of how a unique biobank set up 40 years ago is still reaping scientific fruit today in unimagined ways, through international collaboration.”

Dr James Findon from King’s College London, who was not involved in the research, said: “This a scientifically robust study, but we need to see replication in an independent cohort to really trust the results.

“There is hardly any work on prenatal oestrogen in autism, so while this is a good first step, we have to be cautious and not get carried away.

“It is unknown exactly how prenatal sex steroid hormones may impact neurodevelopment. But this study lays the groundwork for future studies to investigate this relationship further.”

The Independent

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