Rare infections that can be transmitted from a pregnant woman to her developing fetus are associated with a significantly elevated risk of autism and intellectual disability later in childhood, according to a comprehensive new study conducted by researchers at the Karolinska Institutet. The findings, published in the journal JAMA Pediatrics, provide some of the most robust evidence to date regarding the long-term neurodevelopmental consequences of these prenatal exposures.
The research focuses on a specific group of pathogens known collectively as TORCH infections. This medical acronym encompasses a variety of infections—including cytomegalovirus, rubella, toxoplasma, and herpesvirus—that are known to have the potential to cross the placental barrier. While the majority of infections encountered during pregnancy do not reach the fetus, these specific pathogens can penetrate the placenta, leading to a congenital infection that may interfere with the critical stages of fetal brain development.
"It is important to emphasize that it is unusual for these infections to be transmitted from mother to child," says Renée Gardner, a researcher at the Department of Global Public Health at Karolinska Institutet, who contributed to the study. "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."
A Massive Longitudinal Analysis
To understand the long-term impact of these infections, the research team conducted a massive study using Swedish national health and education records. The scope of the project was expansive, covering 3.7 million individuals born in Sweden between 1987 and 2021. By following this large cohort for up to three decades, the researchers were able to link early-life medical history with clinical outcomes that often do not manifest until school age or beyond.
Within this massive dataset, the researchers identified 975 people who had been diagnosed with a congenital TORCH infection. This allowed for a direct comparison between children who had been exposed to these pathogens in utero and the general population, enabling the team to calculate the relative risk of developing specific neurodevelopmental conditions.
The results of the analysis were striking. Children who had experienced a congenital TORCH infection were found to be approximately three times more likely to be diagnosed with autism compared to their uninfected peers. The association with intellectual disability was even more pronounced, with the likelihood of a diagnosis being more than seven times higher. When the researchers isolated cases of severe to profound intellectual disability, the disparity became even more dramatic, with the risk rising as much as 30-fold.
Balancing Relative Risk and Population Impact
While the statistical increases in risk are substantial at the individual level, the researchers were careful to provide important context regarding the overall burden of disease. Because congenital TORCH infections are rare in the general population, they are not a primary driver of autism or intellectual disability rates on a national scale.
The study estimates that these infections may be associated with roughly 1.2 percent of severe intellectual disability cases in Sweden, and they account for approximately 0.034 percent of total autism cases in the country. However, when looking specifically at a child who has been confirmed to have a TORCH infection, the outlook is different. At the individual level, the researchers estimate that approximately one in five children born with such an infection may later go on to develop autism.
Hugo Sjöqvist, a PhD student and lead author of the study, notes that the magnitude and duration of the study set it apart from previous efforts in the field. "It has long been known that these infections can cause intellectual disability. 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," Sjöqvist explains.
Ruling Out Confounding Factors
One of the significant challenges in epidemiological research is determining whether an observed association is truly causal or whether it is influenced by shared familial or environmental factors. To address this, the researchers utilized a sibling comparison methodology. By comparing children who had experienced a TORCH infection with their own siblings who had not, the researchers were able to control for shared genetic and environmental characteristics within families.
The pattern of increased risk remained consistent even when comparing siblings, which suggests that the link between these infections and neurodevelopmental outcomes is unlikely to be explained away by shared family factors. Furthermore, the researchers investigated potential links to other neuropsychiatric conditions, such as ADHD and obsessive-compulsive disorder, but they did not identify clear, statistically significant associations.
Beyond formal clinical diagnoses, the study also uncovered subtle but meaningful differences in academic outcomes. Even among children who did not meet the criteria for a formal diagnosis of autism or intellectual disability, those with a history of a congenital TORCH infection consistently received lower school grades on average compared to children who had not been exposed to these infections. This suggests that the impact of prenatal infection on the developing brain may manifest as a spectrum of neurodevelopmental difficulties rather than just a binary outcome of diagnosis versus no diagnosis.
Public Health Implications and Prevention
The findings underscore the potential for long-term, structural changes in the brain resulting from prenatal infections. "Our results suggest that certain infections transmitted to the fetus during pregnancy may have long-term effects on brain development," Gardner says. "Although these congenital infections are rare, some of them can be prevented, which makes them important from a public health perspective."
The study highlights the necessity of robust vaccination programs and prenatal screening, which serve as the first line of defense against these risks. The case of rubella serves as a poignant example of the success of these measures. Since the introduction of national vaccination programs in Sweden, the incidence of congenital rubella has essentially dropped to zero. The researchers emphasize that maintaining high vaccination coverage and ensuring that pregnant women have access to proper screenings are vital components of preventing the complications associated with TORCH infections.
By identifying that these pathogens are not merely transient issues but potential catalysts for lifelong neurodevelopmental challenges, the researchers hope to reinforce the importance of prenatal health. While the individual risk is manageable, the aggregate data provides a clear mandate for continued investment in public health initiatives designed to minimize the transmission of these infections.
The research was supported by the Swedish Research Council. In the disclosure statements associated with the study, co-author David Mataix-Cols noted that he has received author’s fees from UpToDate Inc and is a partner in Scandinavian E-Health AB; however, it was clarified that these affiliations are unrelated to the findings or the publication of the research. As the scientific community continues to explore the complex origins of neurodevelopmental conditions, this study offers a significant contribution to understanding the role that prenatal environmental factors play in shaping cognitive and social development.
