Jan 10, 2011 -- Children born within one year of an older sibling may be three times more likely to be diagnosed with autism, according to a new study in the February issue of Pediatrics.
The study calls attention to interpregnancy interval (IPI), the duration between pregnancies, as a potential risk factor for autism. In the past, much focus has been on environmental triggers of autism such as vaccines as opposed to maternal physiological triggers such as the womb environment.
If the new findings are confirmed and proven to be related to maternal depletion of key nutrients such as folate, it may be possible to prevent autism with nutritional supplements, the study authors and autism experts suggest.
The latest statistics from the CDC show that one in 110 children in the U.S. has an autism spectrum disorder. This is the umbrella term for a group of developmental disorders that can range from mild to severe and that often affect social interaction and communication skills. According to information in the new study, the proportion of births occurring within two years of an earlier birth increased from 11% to 18% between 1995 and 2002.
Researchers analyzed autism risk among more than 660,000 second-born children born in California from 1992 to 2002. Those children who were conceived within one year of an older sibling were more than three times as likely to be diagnosed with autism when compared to peers who were conceived more than three years after the birth of an older sibling.
Children conceived 12 to 23 months after an older sibling were nearly two times more likely to be diagnosed with autism; children conceived two years to 35 months following an older sibling were one and a quarter times more likely to be diagnosed with autism, the study showed.
The findings held even after researchers took into account other factors that may be related to closely timed pregnancies, such as maternal age and maternal education.
“The robustness of the findings was really shocking,” says study author Peter Bearman, PhD, the Jonathan Cole Professor of the Social Sciences at Columbia University in New York City.
No Consensus on How Closely Spaced Pregnancies Affect Autism Risk
Still many questions remain, including exactly how closely spaced pregnancies may affect autism risk, Bearman says.
“It could be a biological factor, such as maternal depletion of nutrients like folate, or another process that hasn’t been described or discovered yet,” Bearman says. “If the mechanism is depletion of nutrients like folate, then women can make sure to take supplements of it, and if it is something else, it also may be readily modifiable.”
It also could just be that parents with closely spaced kids are more attuned to normal child development, he says. “Parents who have had closely spaced children are more aware of developmental dynamics and more likely to seek help if the child is not developing on the right trajectory,” he says.
“Watch the science,” he says. “This is the first study, but there is a lot more work to be done.”
Autism Community Reacts
The new findings are “significant and intriguing,” says Andy Shih, PhD, vice president of scientific affairs at Autism Speaks. “It is not a simple picture."
“There has been a lot of attention on environmental postnatal factors of autism, but this study suggests that when you have children too closely together, you may not be providing an optimal womb environment,” Shih says.
Other studies have shown that pregnancy complications, low-birth-weight babies, and premature births may also be associated with increased risk for autism, he says.
“The real exciting aspect is the focus on prenatal risk factors, and it points to venues of new research to identify what is happening in shorter IPIs as opposed to longer ones,” he says. “We need a lot more follow-up to make sure we are seeing something that is robust and real."
“If you are worried or concerned, work with your obstetrician/gynecologist on your childbirth and reproductive strategy,” he says. “As time progresses, we’ll soon know if this is something to think about in regard to family planning.”
Dan Khoury, MD, chief of the section of developmental and behavioral pediatrics at Nationwide Children’s Hospital in Columbus, Ohio, urges people to keep the new findings in perspective.
The absolute risk of autism, 1%, is still on the low side, he says.
“Closely spaced pregnancies increase the risk of autism, and that is really all we can say now,” he says.
“Pregnancy is very taxing on the mother and depletes a variety of nutritional stores, including iron and folate, so one theory is some deficiency of nutritional stores is at play, and if, in fact, that is what is causing this increased risk, we think we may be able to treat it through supplementation,” he says.
Fred R. Volkmar, MD, the Irving B. Harris Professor and director at the Child Study Center at Yale University in New Haven, agrees that the findings need replication before any conclusions can be drawn.
“This is preliminary, and nobody else has seen this yet,” he says. “We don’t know if it is true, or if it is true, why would it be true.”
Autism risk aside, many challenges exist for parents who have children so close in age, he says.
“The party line used to be to wait three years between pregnancies, and that is still not bad advice,” he says.
“It is too early to make a recommendation, but be aware of this,” says Gary Goldstein, MD, president and CEO of the Kennedy Krieger Institute in Baltimore. “The data is striking, and I was really taken by this study, but this is a new thought."