In short, the researchers matched every single sibling birth in California from 1992-2002 with reports of getting services for autism, spent a good deal of time on the statistics and did a secondary case-controlled study to make sure they weren't missing anything. And turns out that the odds of a second child born within 12 months of a first child (*actually I messed this up a little - it is an interpregnancy interval of less than 12 months - so a second child born less than 18 months after the first. sorry!) have a little more than a 3 fold risk of having autism than a second child born more than 3 years after the first one. Risk for second children born at interpregnancy intervals of between 12 and 36 months were middling, but risk rose abruptly between 12-18 and "0"-12 months.
Unfortunately, the researchers spent so much time in the paper reviewing the statistics and making sure every last variable was accounted for that the discussion as to why was about one paragraph. They thought it might be folate depletion, omega 3 depletion, or stress (it is, obviously, very stressful to have a young baby and to be pregnant at the same time.) In a number of posts on autism I have speculated as to some nutritional and genetic causes:
Diet and Autism1
Diet and Autism 2
Autism and Vitamin D
Autism 4 - Inflammation Speculation
Brain Efficiency, Pediatric Edition
Since, during pregnancy, a baby will tend to suck whatever nutrients are needed straight from mama, whether she can spare them or not, it would make sense that a nutritional explanation could account for the increased risk of autism in second children when the pregnancies are closely spaced.
Here is a free, online, up to date, and comprehensive review of pregnancy, nutrition, and birth outcomes - if you are interested.
My ultimate preventative solution is, of course, to make sure that moms-to-be out there are consuming nutrient rich diets with plenty of folate, phospholipids, minerals, omega 3s, etc. etc. etc.
Also, it is snowing. A lot.
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