Retinoic acid is a metabolite of vitamin A.  It would seem that oral consumption of vitamin A as well as vitamin D could influence levels of it.  There have been significant increases in the levels of vitamins A and D during the last couple of decades among babies thanks to increased supplementation via vitamin drops and increased fortification.  Large doses of these vitamins given to baby rats induce life-long effects on neuronal development.  Here is a paper on this: http://www.ncbi.nlm.nih.gov/pubmed/19053013

Perhaps it would make sense to devote research effort to determining if the significant vitamin supplementation and fortification that usually accompanies infancy could be a factor in autism as well as the chemical factors being examined.