The goal is to be aware of these concerns before you know they are your child's concern, and prevent the issues or augment at a very early age.
Mothers could take probiotics and/or limit antibiotic use, before birth. They could eat more Omega-3 foods or supplements and make sure their child gets Omega-3s. They could stop smoking during pregnancy and for the earlier years of the child, afterward. They could filter out PFAS from their water and minimize products with them, including some lubricants and fire-retardant materials like some non-stick pots and pans. It also probably wouldn't hurt to have better air and vacuum filtration options, too, perhaps approaching HEPA quality.
These are things that come to my mind.
A fecal transplant might help with the bowel problems that many suffer with autism.