Did you have amalgam fillings replaced while preggo orbfing your ASD/SPD kiddo?
I actually didn't remember having any work done while I was pregnant, but when she was around 18 months and not caught up by a long shot (she was born 2 months premature due to pre-eclampsia), I asked my dental office if I'd had work done. It turns out that I'd had a very large amalgam filling removed and another put in about a month before my daughter was born. She's been autistic since birth (ie not a regressor). There's no other autism in my family.
Autism is not caused by environment or toxins. In fact, a recent study has pointed to a genetic origin as 1% of boys on the spectrum can be found to have a chromosomal defect.
Certainly mercury isn't good for you and those fillings should be replaced over time. But they are not going to "give" your child autism.
While there is almost definitely a genetic component which can make one susceptible to developing autism, most experts agree that there's most likely an environmental insult that triggers it. A purely genetic cause would not explain the epidemic that we're currently experiencing. And both the gene affected and the environmental insult can be different for different children, which is why it's so difficult finding causes for it.