A short time period between pregnancies may increase a later-born child's risk of autism, according to a new study.

The results show second-born children were three times more likely to have autism if they were conceived less than 12 months after their older sibling was born.

The more time that had passed between the birth of the first child and the conception of the second, the lower the second child's risk of autism was, the researchers said.

The results provide clues to possible autism risk factors , said study researcher Keely Cheslack-Postava of Columbia University in New York. However, it's too early to advise women hoping to lower their child's autism risk to space out their pregnancies over wider intervals, she said.

The researchers said they can only speculate about the cause of the link. And it's possible that having two children similar in age simply means parents more readily recognize developmental problems in their second child (the second child's behavior is abnormal compared to the first), and thus these children are more likely to be diagnosed with autism.

But a biological factor might also be responsible. For instance, mothers might have low levels of essential nutrients, such as folate, if they conceived a child shortly after giving birth to one because the first pregnancy diminishes these nutrient levels, the researchers said.

Future research will need to untangle these potential causes and make sure additional factors specific to the families are not responsible for the link, the researcher said.

Second pregnancies and autism

The time between the birth of one child and the conception of another is known as the interpregnancy interval. Short interpregnancy intervals have been associated with conditions such as premature birth, low birth weight and even schizophrenia. The new study is the first to look at the link between the interpregnancy intervals and autism, the researchers said.

Cheslack-Postava and her colleagues examined the birth records of more than 660,000 sibling pairs born in California between 1992 and 2002. The researchers only included pairs in which the firstborn sibling did not have autism, because it's possible that parents may wait longer to have a second child if their first child had autism.

Second-born children conceived after an interpregnancy interval of less than 12 months were three times more likely to have autism than second born children conceived after an interpregnancy interval of three years or more, they study found.

If the interpregnancy interval was 12 to 23 months, the second-born children were 1.8 times more likely to have autism than those born after an interpregnancy interval of three years or more. And if the interpregnancy interval was 24 to 35 months, second born children were 1.26 times more likely to have autism than those three or more years after the previous child. The results held regardless of the mother's age.

But the risk might be specific to certain families, the researchers said. It could be that families with children that are close in age tend to have additional factors that boost the risk of autism. To account for this, the researchers also analyzed nearly 6,000 sibling pairs in which one child had autism.

In this sample, second-born children conceived after an interpregnancy interval of less than 1 year were more likely to have autism compared to their older siblings. If the autism risk in the second-born children was due to something inherent to the families themselves, these children would have been expected to have an equal risk for autism, the researchers say.

The researchers noted the study was based on children living in just one state, and so the results might not necessarily apply to other regions.

An increasing trend

The findings are particularly important in light of data showing that an increasing number of women are giving birth to children just a short time after a previous pregnancy , the researchers said.

In 1995, 11 percent of U.S. births occurred within 24 months of a previous birth. By 2002, the proportion of these births had increased to 18 percent, the researchers said.

Confirming the link between autism and interpregnancy interval is important, Cheslack-Postava said, because if further evidence is found, in some cases, women might choose to wait longer between pregnancies. The study is published in the February issue of the journal Pediatrics.

Pass it on: Having one child too soon after another might increase the second child's risk of autism.

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