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Increased Autism Risk Found in Closely Spaced Pregnancies


An examination of California birth records found second-born children were more than three times more likely to be diagnosed with autism if they were conceived within 12 months of the birth of their older sibling. The farther apart pregnancies were spaced, the lower the risk of autism.

The study, “Closely Spaced Pregnancies Are Associated With Increased Odds of Autism in California Sibling Births” published in the February 2011 issue of Pediatrics (published online Jan. 10) examined the odds of autism among more than 660,000 second-born children. Compared to children who were conceived more than three years after the birth of an older sibling, children conceived after an interpregnancy interval (IPI) of less than 12 months were over three times more likely to be diagnosed with autism. Children conceived after an IPI of 12 to 23 months were almost two times more likely to have been diagnosed with autism, and children conceived after an IPI of 24 to 35 months were one and a quarter times more likely to have been diagnosed with autism. One possible explanation for the increased risk of autism is that women are more likely to have depleted levels of nutrients such as folate and iron, as well as higher stress levels, after a recent pregnancy; however, these factors were not tested in the current study.  

Study authors suggest the finding is particularly important given trends in birth spacing in the U.S.; between 1995 and 2002, the proportion of births occurring within 24 months of a previous birth increased from 11 percent to 18 percent. Closely spaced births occur because of unintended pregnancies but also by choice, particularly among older women who delay childbearing. The study was funded by the NIH Director’s Pioneer Award Program.


The American Academy of Pediatrics is an organization of 60,000 primary care pediatricians, pediatric medical subspecialists and pediatric surgical specialists dedicated to the health, safety and well-being of infants, children, adolescents and young adults. For more information, visit