There is a lot of research that supports spacing 21/2-3 years apart. Among many reasons, traditionally, has been a link to underweight in subsequent babies.

Apparently there is now some linking to autism risk (a link to a report by CNN as I have not been able to find the original study, being in a rush)
Closely spaced pregnancies increase autism risk – The Chart - CNN.com Blogs

Autism is on ASD.

Note that statistical data are accumulated over large samples.