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 http://ods.od.nih.gov/factsheets/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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