Hi,
This is something we're also going through. My 3.4 year old has Asperger's and is hard work but wonderful. We have always wanted 2 children and have put off trying due to the stresses and worries of life with DS, but despite many sleepless nights pondering the whole issue, we decided to go for it. I became pregnant after several months (am no spring chicken!) but had an early MC.
This threw me even more as I didn't dare get too excited about the PG as have had previous MC, but when it didn't work out I almost felt a tiny bit of relief that the worry was over, followed by overwhelming guilt at that relief, small as it was.
We have no local family or anyone who can help when we have a bad night etc. so I do worry we would be taking on too much if it were to work out for us again. I worry that DS wouldn't cope well with a sibling (he's jealous of any attention I give other children) and that we should be satisfied with the gorgeous little boy we have.
However, I also worry that we'll regret it if we don't have (or at least try for) another one, I think that it would be good for DS socially to have a sibling, and he will be off to school by the time another came along, which would possibly make things easier during the day, unless he can't cope AARRGGGHHH!!!
You see? It's not at all straightforward.
And all this is without the possibility of the second child having SN. Or the fact I seem to spend my life at the GPs these days.
I guess I've typed this essay to show you that you're not alone in this situation, but I have no answers for either of us!
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Good luck, whatever you decide!