BRIEFS

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.

The results:

  1. 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:
  2. 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.
  3. 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

ResearchBlogging.org

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7 Responses to Maternal infection during pregnancy and autism: The “flu hypothesis” revisited.

  1. JulieL says:

    Nestor, so is the take home message that any infection is the main concern in the first two trimesters rather than focusing on influenza? What I can’t figure out is how to compare the increase RISK of ASD and infection (200% and 42%) vs the 50% POPULATION of mothers with influenza and ASD children.

    Thanks.

    • Hi Julie, actually the position of the authors is the opposite. They think the effect of infection during the first trimester is mostly due to influenza. So they argue that the focus is on influenza rather than infection itself. Regarding your second question, let’s focus on the first trimester finding. Infection during the first trimester resulted in a 200% greater risk for ASD. Out of those kids (ASD and 1st trimester infection), 50% had mothers whose infection was due to influenza. Therefore, the authors concluded that the risk observed in kids of mothers who had infections during the first trimester is likely driven by influenza.

      Now, here is another piece of data that puts this in 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.

  2. JulieL says:

    Ok, your ending stats sum it up for me. I did see that you concluded that the authors focused on influenza, yet the RISK vs POPULATION which essentially use two different stats was difficult for me to compare. I could not tell if their conclusion supported the stats.

    • Hi July, but just to clarify. we can’t dismiss high relative risk findings based on low attributable risk. That is, the fact that most ASD cases are not attributable to infection during the first trimester is statistically and conceptually unrelated to the finding that infection during the first trimester increases the risk for ASD. These are two different processes. For example, exposure to nuclear radiation has a massive impact on your risk of developing cancer, yet +99%most (i actually don’t have the actual number so ‘most’ is accurate enough) of cancers are not attributable to exposure to nuclear radiation. One finding does not impact the validity of the other, as they are mostly statistically and theoretically unrelated and speak to two completely different questions. n.

  3. JulieL says:

    I see what you are saying. I agree, to a know possible, plausible reason for some ASD cases is in fact important, even though most ASD cases are not in result of influenza. The fact that there are many theories (genetics, toxins, etc) as to how ASD has been caused, does not negate the fact that many or all may have some responsibility/attributed to ASD cases. Thanks for clarifying.

  4. DSC says:

    I doubt that infuenza really has anything to do with it. What medicines did the woman take for the illness? How many got vaccinated during pregnancy and got ill from the vaccination?

    People have pointed out that since fetal tissue has been used in vaccinations that the autism rates have gone up almost exponentially. Especially for children born in 1988 and after.

  5. Neuroskeptic says:

    “Out of those kids (ASD and 1st trimester infection), 50% had mothers whose infection was due to influenza.”

    Doesn’t that just reflect the fact that influenza is the most common viral infection in the West? (well apart from the common cold, but that’s hard to assess in retrospect.)

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