The results of last week’s poll are in and the autism article won by a wide margin. So here are some thoughts on the recent pregnancy timing and autism study.

A team from Columbia University was interested in examining the link between Inter Pregnancy Interval (e.g., time between pregnancies; IPI) and autism. IPI is important because short intervals between pregnancies (e.g., having kids too close together) is associated with specific physiological factors that have been linked to developmental problems, such as low birth weight, prematurity, etc.

For the study, the researchers examined all births in California from 1992 to 2002. They were able to identify 662,730 sibling pairs for which they had information about the timing of the pregnancies. That is, they identified the number of months between the births of the first and the second sibling. They then obtained a large number of demographic and clinical information such as race/ethnicity, gestational age of the pregnancies, paternal age, etc. They were also able to gather information about the autism diagnosis of these children from the Department of Developmental Services (DDS) records.

A total of 3,137 second-born siblings with autism were identified. An important characteristic of this study is that the DDS does not provide services to children with only PDD-NOS or Asperger’s. Therefore the results are specific to the presence of full autism.

The authors then calculated whether the probability of having autism among second-born siblings changed as a function of the number of months that lapsed since the birth of the first-born sibling.

Below you will see a graph that summarizes the very surprising results. On the horizontal axis you see the IPI in months. That is, the number of months between the birth of the first and second-born siblings. On the horizontal axis you will find the odds ratio (a type of probability) that the second-born child will have autism. The line in the middle across the odds ratio of 1 reflects a 50-50 chance that the second-born child will have autism. When the odds ratio is above this line, it means that the probability of autism increases. When the odds ratio is below the line, it means that the probability of autism decreases.

As you can see, the probability that the second-born child had autism was very high if the time between pregnancies was under 12 months. In contrast, the probability was lower if the time between pregnancies was longer than 24 months.

The numbers below give you an even better picture. Once you control for a number of factors such as child’s sex, parental age, etc etc, having pregnancies close together greatly increased the risk for autism in the second-born child. Specifically:

- Children born less than 12 months after their siblings were close to 300% more likely to have autism when compared to second-born children born 48 months after the first sibling.

- Children born between 12 and 23 months after their siblings were 110% more likely to have autism when compared to second-born children born 48 months after the first sibling.

- Children born between 24 and 35 months after their siblings were 42% more likely to have autism when compared to second-born children born 48 months after the first sibling.

The risk finally stabilized at 36 months. Specifically, being born 36 months after their siblings did not increase or decrease the chance of autism as compared to kids born 48 months after their siblings. Likewise, being born many many months after their siblings (for example more than 84 months) did not reduce the chance of having autism as compared to those born 36 months after their siblings.

This suggests that waiting 36 months between pregnancies would reduce the risk of autism but waiting longer provides no added benefit.

Now the really interesting question is why. What is the mechanism that could explain this finding?

The authors suggest that a likely cause may be folate depletion. Short time between pregnancies is associated with nutritional depletion and folate depletion in particular. Folate is a critical nutrient needed during pregnancy for DNA synthesis and levels of maternal folate decline drastically during the 12 months after having a child.

Here is an useful article from the National Institutes of Health about folate

The Reference: Cheslack-Postava, K., Liu, K., & Bearman, P. (2011). Closely Spaced Pregnancies Are Associated With Increased Odds of Autism in California Sibling Births PEDIATRICS, 127 (2), 246-253 DOI: 10.1542/peds.2010-2371

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12 Responses to Pregnancy timing and autism risk: Wait 3 years between births to lower your child’s risk?

  1. [...] This post was mentioned on Twitter by, Christa Nieuwboer. Christa Nieuwboer said: “@ChildPsychology: Pregnancy timing and autism risk: Wait 3 years between births to lower your child’s risk?” [...]

  2. Thanks.

    If the explanation is nutritional depletion, did the study say anything about if it is possible to remedy this, for instance with suplements, changing diet and so on, to reduce the risk even if one have children at a shorter interval?

    • @Hans, folate supplementation is very common among pregnant women. for example, March of Dimes recommends folate supplementation to pregnant women. However, there is data yet that suggests such supplementation can alter the risk for autism.

      @Nancy, likely the cause of the recent increases are unrelated to this specific risk factor. There are multiple risk factors and unfortunately I doubt that we will ever find one answer that explains what causes autism and what contributes to the recent increases.

  3. Nancy says:

    My 2nd son has autism, born more than 36 months after my first son. So although he doesn’t fall into line with this study, the findings are certainly interesting. It’s so difficult to figure. How would the significant increase in diagnosed cases of autism factor in? It seems that people are having less children … and with more time between (overall). Just not sure this is the answer.

  4. Canoemom says:

    Did they factor in whether and for how long the first child was breastfed? Wouldn’t that also influence maternal nutrition levels?

  5. Kevan Gelling says:

    If the explanation is nutritional depletion, rather than folate, could vitamin D be the depleted nutrient? Vitamin D varies by season, latitude and race. Did the researchers look at these factors? Was the autism / 2nd pregnancy link stronger for spring births, say?

  6. Judy says:

    My 1st child has autism, but PDD-NOS, so he would not be
    Tracked in the study. His sister, 17 months younger is nt.
    3rd child, 22 months younger also has PDD-NOS. That being
    Said my 1st child was only a few months after a miscarriage.
    All very interesting, and great that there is so much data in the study.
    Too bad that PDD-NoS is not included, and other factors not
    Tracked by DDS.

  7. Judy says:

    I would be curious about a breastfeeding and/or maternal nutritional depletion study as well. One might think that there would be an inherent gap increase between pregnancies for mothers that exclusively breadtfeed for an extended time.

  8. Astrid says:

    Interesting study. I am the oldest living child and I have autism, but my mother had a stillbirth a year before I was born. I would assume that would have the same effect, cause folate depletion would happen whether the child is born alive or not.

    • kathy says:


  9. Stacey says:

    I have 3 children aged 10,11,12 my middle child is very withdrawn and shy she experience extreme meltdowns but doesn’t fit any other of the other criteria of ASD my youngest child has so many signs and I have been told by his teachers from many schools (due to his behaviour) tell me this is what they think it is but I have never been able to get any support so I have done all the research I can and have even used techniques that have been recommended by other parents with AS children including social interacting it tookme 2 years to train my son to look at people when he was speaking and being spoken too he is also very social with people that are playing sports as this is his interest. By me doing this it has now meant that they are saying he doesn’t have AS because he is social. How do I get them to listen to me as a parent.

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