1) I have one dd with autism and one without. The younger one is autistic, never vaxed, no ultrasounds. The older dd had at least 3 maybe 4 US, and was fully vaxed up until 18 months.
2) Some of us don't really feel like autism is something terrible. Just to keep that in mind when discussing causes/prevention.
3) I think sometimes mercury or other toxicities and can cause brain damage and seizure disorders (like what Jenny McCarthy's son had) and that there are metabolic disorders can mimic autism but are not necessarily the same as a child from a family where ASD and Aspie tendencies are part of a genetic pattern... which, as it turns out is the case in my family. But we are only figuring that out with 20/20 hindsight. So a GFCF diet can seem to "cure" their autism, or chelation therapy or what have you. Because they weren't really autistic to begin with. My tin hat theory of the day.

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Anyway. My two cents for what it's worth. I am still on the fence about a third child but it wouldn't matter worth a moth's fart if the child was autistic or not, so it's not like "oh no it runs in the family, I had better adopt."
If anything, parenting my ASD child (and granted, my older dd is probably mild asperger's but is undiagnosed and no plans to have her evaluated) has helped me get in touch with and embrace my own Aspie tendencies and is helping with my own personal growth. I'm sure I'll post more about that journey at some point in a more suitable thread.
-Vijay