A new study from Northern California says that children born less than 21 months, or more than 6.5 years, after their first-born siblings had a higher risk of autism. The researchers do not know what could have caused their autism rates to be two to three times higher than children who were conceived three to four years after the birth of a first child.

The researchers from Kaiser Permanente noted that a mother's weight, her use of antidepressant medication, parents' ages, and problems during either pregnancy did not explain the results.

They examined data from 45,261 children born between 2000 and 2009 who were members of the Kaiser Permanente Northern California (KPNC) health care system. The study results were published online in the journal Pediatrics this week.