There is some preliminary research suggesting that there is a relationship between birth spacing and autism, that the closer babies are spaced, the more likely it is the 2nd baby will have autism. This is very interesting research, but I am inclined to be skeptical, for now.
Although the study was done on half a million children, the number of children diagnosed with autism was quite low.
The overall prevalence of autism was less than 1 percent in the study. Of all the 662,730 second-born children in the analysis, 3,137 had an autism diagnosis. Of the 156,034 children conceived less than a year after the birth of their older siblings, 1,188 had an autism diagnosis — a higher rate, but still less than 1 percent.
Less than 2,000 second children had a diagnosis of autism. No rate of autism was higher than one percent. It seems to me that with a rate so low, it would be very easy for a confounding factor to appear.
The researchers themselves mentioned that closely born 2nd children might be diagnosed at a higher rate because their parents are paying more attention to their development and comparing it to the close-in-age sibling’s development.
In conclusion, this is very interesting research, but I will not put too much stock in it until some confirming studies have been done.