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msnbc.com contributor
updated 1/10/2011 8:19:32 AM ET 2011-01-10T13:19:32

Parents planning more than one baby may have another reason for giving extra thought to the timing: A new study shows that the risk of autism may go up when a second child is conceived shortly after the first is born.

Columbia University researchers found that the risk of an autism diagnosis in a second-born child rose more than three-fold when the child was conceived within 12 months of the birth of the first baby, according to the study which was published online Monday in Pediatrics.

And second-borns conceived between 12 and 23 months after a first child was born had twice the risk of being diagnosed with autism compared to babies conceived a full three years after an older sibling was born.

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The findings might be a sign that that something in the uterine environment is changed in the years immediately following pregnancy — women might be deficient in certain nutrients, such as folate, for example. But they might also be explained by some other factor shared by mothers who choose to conceive a second child shortly after the first is born, said the study’s lead author, Keely Cheslack-Postava, a post-doctoral researcher at Columbia.

The researcher, as well as outside experts, cautioned that parents should not be too alarmed by the new findings. “At this point we aren’t able to say from this research that delaying a second pregnancy would have an effect on autism risk,” said Cheslack-Postava.

And while the increased three-fold risk for second-borns may sound high at first, parents need to remember that the overall risk of autism is low, said Dr. Rita Cantor, a professor of human genetics at the David Geffen School of Medicine at the University of California, Los Angeles.

“There are a lot of people who have closely spaced pregnancies who don’t go on to have children with autism,” Cantor said.

Insight into environmental factors for autism?
What the study does offer is insight for researchers seeking factors that might increase the risk of autism, experts said.

“We haven’t had too much success in finding environmental factors for autism,” Cantor said. “And if you consider the in utero environment to be an environmental factor — which it is — then this is a good first step.”

For the new study, researchers scrutinized birth records from 662,730 second-born children from California, all of whom were born between 1992 and 2002 and none of whom had an older sibling with autism. By age 6, 3,137 of the second-borns had received a diagnosis of autism, according to data provided by California’s Department of Developmental Services. Of those, 2,747 occurred in children born less than 36 months after their siblings, the study showed.

The birth records included information about the parents such as educational level and age, so the researchers were able to account for a number of factors in their calculations. Even when factors such as parental age and birth weight were included, a shorter length of time between births was still associated with a higher risk of autism.

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Shorter birth intervals tied to other risks
Cheslack-Postava and her colleagues chose to look at birth intervals and autism because earlier research had found that shorter intervals between babies are associated with other brain diseases, such as schizophrenia.

The researchers speculated that second-borns conceived shortly after an older sibling’s birth might end up with developmental issues because of nutritional deficits. Other studies have shown that in the years immediately following a birth, the mother is often low in folate, iron and other nutrients, Cheslack-Postava said.

Autism researchers had hoped they would find one gene that predicted which children would develop autism, said Dr. Carolyn Bridgemohan, a developmental behavioral pediatrician and medical director of the autism care program at the Children’s Hospital of Boston.

“That hasn’t happened,” Bridgemohan said. “There are probably many factors, both genetic and environmental, that occur together that increase the risk.”

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So, should parents err on the side of caution and wait a bit longer before conceiving a second baby?

The World Health Organization recommends that for having healthy babies, women not attempt another pregnancy until after 24 months following childbirth, said Elizabeth Clark, a spokesperson for Planned Parenthood. Studies based on data in Americans lead to a less conservative recommendation — 18 to 23 months, Clark said.

In addition to the autism risk suggested in the new study, there are other issues, such as a higher risk of prematurity, that become more likely if a second child is conceived shortly after the first, Bridgemohan said.

“There are a variety of reasons to think it would be better to have a larger spacing between pregnancies, for both the mother’s health and the prenatal health of the developing baby,” she added. “Still, I don’t know that we should take this study as saying you must wait or you’ll be doing harm to your child.”

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