Jan 12, 2011

Short Interval Between Pregnancies Linked to Increased Autism Risk

Children born at shorter intervals between pregnancies are at increased risk of developing autism, according to new research published online January 10 in Pediatrics.

“Second-born children who were conceived less than 12 months after their sibling’s birth were at well over 3 times the odds of an autism diagnosis vs those who were conceived 3 or more years later,” lead study author Keely Cheslack-Postava, PhD, a Robert Wood Johnson Foundation Health and Society Scholar and a postdoctoral fellow at Columbia University, New York City, told Medscape Medical News.

“There was a decreasing relationship, so the highest risk was observed in those who were born less than 12 months after their sibling, but if they were born 12 to 24 months they were almost twice as likely to have an autism diagnosis vs those who were conceived 3 or more years later,” she said.

Other studies have shown that a short interpregnancy interval (IPI) is associated with adverse outcomes, including preterm birth, low birth weight, and schizophrenia. The Columbia University group focuses on autism research and therefore sought to determine whether there might be a relationship between the IPI and risk for the disorder.

They analyzed birth records of all children born in California between 1992 and 2002, focusing on first- and second-born, full sibling, singleton births, as well as autism diagnoses from California’s Department of Developmental Services, which provides services to children in that state who have autism and other developmental disabilities.

Among the 662,730 second-born children in the analysis, there was an inverse association between IPI and odds of autism, the analysis showed.

Table. IPI and Odds of Autism Relative to IPIs of 36 Months or Longer


IPI, mo Odds Ratio (95% CI)
<12 3.9 (3.00 – 3.82)
12-23 1.86 (1.65 – 2.10)
24-35 1.26 (1.10 – 1.45)


CI = confidence interval; IPI = interpregnancy interval

Preterm birth or low birth weight did not mediate this association, and it persisted across categories of sociodemographic characteristics, Dr. Cheslack-Postava said.

“Finding that the sooner the conception occurred following the birth of a sibling the greater the likelihood of an autism diagnosis in the second sibling is important because this interval between pregnancies is a potentially modifiable factor,” she said in an interview.

“We don’t know what about that interval matters for autism, but this is an important clue that may let us learn about potentially modifiable risk factors for autism. We are hoping that others will try to test and see if this shows up in other populations and perhaps get at what’s behind this finding. It was a very strong and robust association.”

Commenting on the study for Medscape Medical News, Susan L. Hyman, MD, associate professor of pediatrics at the University of Rochester Medical Center, New York, said it clearly shows the link between a short interpregnancy interval and autism risk and offers the IPI as an important clue to examine in future studies.

“These types of epidemiological studies are very important. They tell us what questions to ask going forward in prospective studies,” said Dr. Hyman, who is also chair of the Autism Subcommittee of the American Academy of Pediatrics.

“This study alerts pediatricians to be especially alert in their screening for autism and other developmental disabilities in the second-born sibling of a short interpregnancy interval,” she said. “Pediatricians and other child healthcare professionals are the first line of identification for early treatment.”

She warned that the message from this study might be a cause of concern and alarm for families.

“I don’t want to scare families. My children are 16 months apart, and there are a whole lot of people like me out there and when you see things like this you wonder, was I putting my child at risk?” she said.

“And the answer is, well, you know, you do what you do. Pregnancy happens. What this tells pediatricians is to counsel families who find themselves with another baby on the way. Make sure the mothers take care of themselves, get good prenatal care, eat and sleep right, and do not worry because stress is bad. But there are a lot of families who are going to read this and take a second look at their second child.”

Dr. Hyman noted that autism is increasing and there is still a lot of work before we can pinpoint exactly why.

“That’s one of the reasons why these epidemiologic studies are so tremendously important. If people don’t look at the data that has been collected, we won’t know the questions to ask,” she said.

This study was supported by the Robert Wood Johnson Foundation Health and Society Scholars Program and the National Institutes of Health Director’s Pioneer Award Program. Dr. Cheslack-Postava and Dr. Hyman have disclosed no relevant financial relationships.

Pediatrics. Published online January 10, 2011.

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