These hormones, which play a key phase in brain development at three to four months of pregnancy, may also explain why the condition is far more common among males than females, they said.
But it was too early to say whether higher hormone levels were a cause of autism, the team wrote, and cautioned against hormone screening or treatment based on their preliminary findings.
Scientists at the University of Cambridge in Britain and Denmark's Statens Serum Institute analyzed hormone levels among nearly 20,000 stored samples of amniotic fluid, which surrounds the fetus in the uterus.
The team measured levels of four "sex" steroid hormones -- testosterone, progesterone, 17-alpha-hydroxy-progesterone and androstenedione -- which are known to play a role in brain development.
They also looked at a fifth hormone, cortisol, which is a marker of stress.
The researchers found higher hormone levels in the amniotic fluid in 128 males who were later diagnosed with autism spectrum disorder.
All autistic subgroups were found to have the signature: Asperger syndrome, classic autism or a category called unspecified pervasive developmental disorder.
"This is one of the earliest non-genetic biomarkers that has been identified in children who go on to develop autism," said Simon Baron-Cohen, a University of Cambridge professor.
"We previously knew that elevated prenatal testosterone is associated with slower social and language development, better attention to detail and more autistic traits.
"Now, for the first time, we have also shown that these steroid hormones are elevated in children clinically diagnosed with autism. Because some of these hormones are produced in much higher quantities in males than in females, this may help us explain why autism is more common in males."
It was not known what causes the higher steroid levels in the first place, and the team cautioned against using the findings as a tool to screen for autism.
Nor should drugs be used to block steroid hormones.
"This could have unwanted side effects and may have little to no effect," said Baron-Cohen.
Only males were tested in the first phase of research. The next step will be to see if a similar telltale exists for females.
"Steroid hormones are particularly important because they exert influence on the process of how instructions in the genetic code are translated into building proteins," co-researcher Craig Brierley said in an email exchange with AFP.
The study appeared in the journal Molecular Psychiatry.
The cause of autism, a complex neurodevelopmental disorder characterized by social withdrawal, is considered to be roughly split between genetic and environmental factors, according to a study last month in the Journal of the American Medical Association (JAMA).