Illnesses and conditions

Rare placenta-crossing infections triple risk of autism in children

By

Mike McRae

September 24, 2026

Rare placenta-crossing infections triple risk of autism in children

Infections that can cross the placenta can affect a child's neurological development.

(janulla/Getty Images/Canva)

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Infections that can cross the placenta can affect a child's neurological development.

(janulla/Getty Images/Canva)

Suspicions of a link between extremely rare congenital infections and the development of autism spectrum disorder (ASD) have been supported by a massive new study led by scientists from the Karolinska Institute in Sweden.

Previous researchon pregnant mothers has hinted at a modest association between neurological development and the transmission ofpathogens across the placenta, yet due to the nature of the investigations, inherited conditions couldn’t be ruled out.

Using a nationwide register of birth records and medical data, researchers have looked at the same question from a population perspective, comparing the development outcomes of just under 3.7 million individuals born in Sweden between 1987 and 2021.

Collecting together such a large pool of data allowed the researchers to amass a small but significant sample of 975 children who had been exposed to pathogens such astoxoplasmosis,syphilis,rubella,cytomegalovirus, orherpes simplexwhile still in the womb.

Ordinarily, the placenta does a magnificent job of keeping microbes away from a developing fetus. Even those exceptionally well-suited to slipping past the defences,like the rubella virus, are thankfully now rare in modern society thanks to vaccination programs.

Yet in rare instances, pathogens can establish themselves in the unborn’s environment and cause havoc.

The question is, is there a chance this havoc can result in changes to an infant’s neurological wiring that we might recognize in the form of autism, ADHD, or obsessive-compulsive disorder?

“It has long been known that these infections can cause intellectual disability,” says statistician and lead author, Hugo Sjöqvist. “However, the link to autism has been less clear in previous research, which has often been based on small patient groups. This study is the largest to date in this field and is based on national register data covering almost the entire population of Sweden.”

The huge sample also gave the researchers room to take into account the impact of other variables, such as the method of delivery and gestational period.

No consistent associations stood out for obsessive-compulsive disorder, and associations between infections and ADHD could also be accounted for by other relationship factors.

Yet for children exposed to any of the suspect pathogens while in the womb, the chance of developing autism jumped threefold. Among those with a profound intellectual disability, the odds skyrocketed, becoming 30 times greater.

Keep in mind, the numbers themselves are small, even if the increased risk appears to be large. The team estimated that their findings imply that around one in every 100 children with a severe intellectual disability can attribute their condition to an infection from toxoplasmosis, syphilis, rubella, cytomegalovirus, or herpes simplex while in the womb.

For those with autism, the number is much smaller – just 0.034% of all cases.

"It is important to emphasize that it is unusual for these infections to be transmitted from mother to child,” says public health researcher and co-author Reneé Gardner. “They account for a very small proportion of all cases of autism in the population, but for those children who are actually affected, we see a clearly elevated risk of both autism and intellectual disability.”

This research was published inJAMA Pediatrics

Source:Karolinska Institute