the positive correlation between vitamin D deficiency during pregnancy and autism has been known for quite some time. One more aspect of this correlation related to the original post:

https://karger.com/neo/article/113/4/305/227927/Vitamin-D-an...

"the vitamin D-deficient infants have a greater brain mass than pups born to vitamin D-sufficient mothers, indicating impairment of appropriate neuronal pruning"

"deficiency early in pregnancy is more likely to affect brain structure during the period of organogenesis whereas the pruning of neuronal cells occurs later in gestation, so later deficiency would likely present with differences in brain size. "

That also dovetails with that Sweden study that autism is more correlated with vitamin D deficiency in the 3rd trimester.