Hi sleepless!
How are you feeling today?
It sounds like your anxiety is just grabbing on to what it can... Mine does that too. You can find out all the statistics and sometimes reassure it but then it finds something else to focus on and once it gets hooked on the trying to prevent things such as autism which we honestly don't know for sure how it works even in this wonderful age of science, in my experience it never ends. I told you in my PM how it's taken me years to accept it is actually my anxiety that's the problem rather than the things Iv feared.
The Group b strep, anxiety, depression, EMCS, vaccines, pollution, EMFs, genetic risk in my family, tooth fillings, ultrasounds and so on that I have spent my last 4 years obsessing over have had NO effect that I can see on my DS. In sharp contrast to my anxiety that has prevented me enjoying many many moments. Some of the things iv had anxiety over probably seem bizarre and utterly stupid to some people, but the fear is real to me and tbh it comes and goes, my DH just humours me with it but he has gone as far as to say that all I have to do is read the news and i alter my life at my own expense therefore he's cutting my access to the Internet to protect me in a future pregnancy.
I know exactly what you mean, it's not really anything to do with fearing having a child with challenges, your child is always perfect regardless of any challenges that come with them, it's EVERYTHING to do with not wanting to be responsible for challenges that child may face and hold the guilt for it based on a false thought that we have some inside knowledge of what will cause something in OUR child despite we don't think it for anybody else's! Autism has a legacy of mum blame, as does schizophrenia but it's not something currently in anyone's control and likely never will be. We know this in our rational minds, but our anxiety doesn't pay mind to rationality. I think anxiety is just a really vulnerable state and once there one anxiety breeds another. The reality of it is that they could announce on the news later today that they've figured out every single cause of autism and by tomorrow, you and I would worry about something different.
I'm trying hard to make peace with the fact that I can either not have another baby and avoid all risks that come with it, or I can have another and accept that my role as a parent means I have to take those risks on my child's behalf but that child won't exist unless I do take them. For some people it's easy to accept, and come to terms with. For people with anxiety we need to train our minds to accept it by actively working on what we feed our thoughts with. It irritates me to know this though! 
Can you set a goal of only feeding positive things to yourself, maybe a day of only enjoyable things? The moment something feels bad to you, stop it and choose another thing to focus on. I found it impossible at first but then my counsellor told me I could have a worry hour once a day and I HAD to worry in that hour so to save up my worries to worry in that hour I chose. Going from positive to fear and worry for an hour is actually much harder than you imagine but I think pregnancy is so susceptible for bringing on anxiety especially if it was difficult conceiving as your already in a state of wondering if it will ever happen for you (I had problems conceiving but conceived naturally with DS and im still in shock tbh!)