Researchers looked at the intervals of birth spacing and risk of autism in a nationwide registry of siblings born from 1990-2004. The birth spacing interval was the time from birth of a first-born child to the conception of a second-born child; the children were full siblings, meaning they had both the same mother and father. Researchers then looked at the rate of autism in the second-born child.
The study included 223,476 pairs of siblings. In mothers who had a second baby less than nine months after the birth of the first child, 0.25 percent of second-born children had autism. In children born at 36 months or more apart, 0.19 percent had autism. The risk of autism in the second-born child was also increased when siblings were spaced nine to 11 months apart.
“Consistent with a previous report from California, [birth-spacing] intervals shorter than one year were associated with increased risk of autistic disorder in the second-born child,” the authors report. “A possible explanation is depletion of micronutrients in mothers with closely spaced pregnancies.”