The last issue of the Journal of Autism and Developmental Disorders included one of the largest examinations of the association between maternal infection during pregnancy and risk for autism. The study was conducted in the Denmark where researchers examined the maternal infection rates during pregnancy and autism diagnoses for all children born in Denmark between 1980 and 2005. ASD diagnosis was calculated by examining the Danish Psychiatric Central Registry, which includes information of all children who received outpatient or inpatient treatment with a diagnosis of ASD during those years. The researchers also examined the Danish National Hospital Registry, which includes information about all hospital admissions in the entire country during that time.
The researchers wanted to know whether children whose mothers had an infection during pregnancy were more likely than their peers to develop an autism spectrum disorder. In addition, they explored whether the nature of the infection (viral vs. bacterial), or the trimester during which the infection occurred, affected the risk of developing an ASD for the child.
- Overall, there was no relation between maternal infection during the entire pregnancy and ASD risk. However, there was a relation between infection at specific trimesters of the pregnancy and ASD risk:
- Viral infection during the 1st trimester of the pregnancy significantly increased the child’s risk of developing ASD. Specifically, children whose mother had a viral infection during the first trimester were about 200% more likely than their peers to develop an autism disorder.
- Bacterial infection during the 2nd trimester also increased the child’s risk of developing ASD but at a much smaller rate. Specifically, children whose mother had a bacterial infection during the second trimester were about 42% more likely than their peers to develop an ASD.
The authors indicated that this effect is likely due to exposure to the influenza virus. Specifically, in 50% of the children who had ASD and whose mothers had a viral infection during the first trimester, the virus was influenza. Further, 4% of children whose mothers were admitted to the hospital during the first trimester due to the influenza virus developed autism. In contrast, only 0.6% of the children born during the period examined developed ASD. The rate of ASD among those whose mothers were admitted to the hospital during the first trimester due to the influenza virus was 6 times higher than in the general population.
Why is this the case? Researchers don’t know for sure but a few theories have been proposed. One theory is that exposure of the fetus to the influenza virus increases the risk for developmental disorders. However, others believe that it is not the virus itself, but the maternal immune response to the virus that is harmful to the developing fetus.
UPDATE: Here is one additional piece of information that helps puts these findings into perspective. ONLY 1.5% of all ASD cases were children whose mother had infections during pregnancy. That is, even though infection during the 1st trimester significantly increased the risk of ASD, 98.5% of all ASD cases could be considered to be completely unrelated to maternal infection during pregnancy.
The reference: Atladóttir, H., Thorsen, P., Østergaard, L., Schendel, D., Lemcke, S., Abdallah, M., & Parner, E. (2010). Maternal Infection Requiring Hospitalization During Pregnancy and Autism Spectrum Disorders Journal of Autism and Developmental Disorders DOI: 10.1007/s10803-010-1006-y
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