Second-born children are more than three times more likely to be diagnosed with autism if conceived within 12 months of the birth of their older sibling.
An examination of California birth records found that second-born children were more than three times more likely to be diagnosed with autism if they were conceived within 12 months of the birth of their older sibling. The farther apart pregnancies were spaced, the lower the risk of autism, according to research published in Pediatrics.
In the study, Keely Cheslack-Postava, PhD, and colleagues from Columbia University in New York, examined the odds of autism among more than 660,000 second-born children to determine whether the interpregnancy interval (IPI) is associated with the risk of autism in subsequent births. Subjects were identified from all California births that occurred from 1992 to 2002 using birth records, and autism diagnoses were identified by using linked records of the California Department of Developmental Services, and IPI was calculated as the time interval between birth dates minus the gestational age of the second sibling.
Compared to children who were conceived more than three years after the birth of an older sibling, those conceived after an IPI of less than 12 months were over three times more likely to be diagnosed with autism. Children conceived after an IPI of 12 to 23 months were almost two times more likely to have been diagnosed with autism, and children conceived after an IPI of 24 to 35 months were one and a quarter times more likely to have been diagnosed with autism.
These results suggest that children born after shorter intervals between pregnancies are at increased risk of developing autism.
According to Cheslack-Postava and colleagues, one possible explanation for the increased risk of autism is that women are more likely to have depleted levels of nutrients such as folate and iron, as well as higher stress levels, after a recent pregnancy; however, these factors were not tested in the current study.
The authors suggest the finding is particularly important given trends in birth spacing in the United States; between 1995 and 2002, the proportion of births occurring within 24 months of a previous birth increased from 11% to 18%. Closely spaced births occur because of unintended pregnancies but also by choice, particularly among older women who delay childbearing.