Closely Spaced Pregnancies Linked to Autism Risk
Autism is a complex disorder and it is likely that a combination of factors is involved, including both genetic and environmental risks. One area of scientific study is that surrounding the pregnancy and birth of autistic children, including such risk factors as parental age, preterm birth, and low birth weight. Researchers from Columbia University have discovered a link between pregnancy spacing – the period of time between the birth of siblings – and an increased risk of autism.
Autism Risk Three Times Higher When Siblings Born Close Together
Over the last 40 years, the prevalence of autism spectrum disorders, including autism, Asperger syndrome and pervasive developmental disorder, has risen 10-fold to about one in 110 children, according to CDC data from 2006.
Peter Bearman, director of the Lazarsfeld Center for the Social Sciences at Columbia, and colleagues focused on over 662,000 second-born sibling children born in California between 1992 and 2002, as recorded by the state’s Birth Master Files. According to data collected by the US Department of Education, California has the highest rate of autism cases of any state in the nation.
For each sibling pair, the firstborn was not diagnosed with autism. In 3,137 of the pairs, the second child was.
Overall, second children of mothers who became pregnant within a year of giving birth to the older sibling, about 7.5 in every 1,000 were diagnosed with autism compared with 2.5 out of every 1,000 when pregnancies were spaced three or more years apart.
The mechanism by which closely spaced pregnancies may boost autism risk remains unclear – the association between sibling spacing and autism risk held true even when other risk factors such as maternal age and low birth weight were controlled.
The authors offered two possible explanations: Autistic behaviors might be more noticeable when there's an older sibling close in age for comparison; or a biological factor, such as maternal depletion of nutrients such as folate, important for brain development, could put the developing fetus at risk.
Closely spaced pregnancies also increase the risk of early delivery and miscarriage.
"We've identified a really robust association," said Bearman. "When you see something so robust and so stable, it provides an important clue as to what we should be looking at next."
Women shouldn’t be alarmed by this study, however, warns Dr. Thomas Insel, the Director of the National Institute of Mental Health.
"This is still an odd and rare occurrence," he said. Although 1,188 children conceived within 12 months after a sibling were diagnosed with autism, 154,846 children were not. "Even with a closely spaced pregnancy, 99 times out of 100 you're not going to have a child with autism.”
Cheslack-Postava K, et al "Closely spaced pregnancies are associated with increased odds of autism in California sibling births" Pediatrics 2011; 127: 246-253.