A shorter time-gap between the birth of a first and second child creates greater odds that the second child will develop autism.
Interpregnancy interval (IPI), the amount of time between births, may affect maternal physiology and, consequently, the fetal environment.
This article presents a study that looked for associations between the duration of IPIs and autism; the data set was all births in California from 1992 though 2002. The authors restricted their research to pairs of first and second born siblings, classifying each pair by IPI (e.g., less than 12 months). Logistic regression models revealed associations between IPI and autism; an adjusted model accounted for potential covariates (e.g., parental age and ethnicity).
From 1995 to 2002, there was a near 100 percent increase in births occurring within 24 months of a previous birth. The authors of this article have identified short IPI as a modifiable risk factor associated with autism.