Star, I've been meaning to reply to this for days.
We have had a similar dilemma. I had twins first time round and as they got older, I felt like our family was not complete. I am one of three so I'm not sure if that was part of it. As it became apparent that DS1 had SN we were concerned about having another child with a similar condition. However, in the back of my mind was the worry that the burden of possibly looking after DS1 was going to rest on DS2 when we were gone. So we started TTC at the start of 2010 and I got pg very quickly but had a miscarriage at 12 weeks. Shortly after that DS1 was diagnosed with autism and DH got cold feet about TTC again for a while.
After DS1 was diagnosed, we were referred to genetics because the community paed was not convinced that it was just autism. DS1 has poor muscle tone and is quite wobbly and so he wanted the geneticists to take a look at him and see if there was anything else that he might have missed. We had already had basic genetic tests for fragile x and all chromosomes appeared present and correct. The consultant said that our risk of having another child on the spectrum is around 5%. If we had a child with "classic" autism, she said it would have been around 10%. 5% was a risk we were prepared to take. She also said that lots of children she sees with ASD have had some birth/pregnancy issues - in our case, I had pre-eclampsia, DTs were very low birthweight and DS1 has a squashed head. I wonder in our case if it was a case of being genetically susceptible and having an environmental insult in utero.
We have 2 very different boys and having DS2 definitely helps DS1 as he copies so much. I think having another will only help that further. DS1 is making good progress and I can see that in a few years time things won't be as difficult as they have been. I also would like DS2 to have an (hopefully) NT sibling.
I am now nearly 16wks PG with #3 and slightly terrified about whether the gamble will have paid off. We are not finding out the sex but I know that if we have a girl, I won't be as worried about ASD as if we have a boy.
Yes we might have another child with ASD (or something else) but we feel that we could cope with another child like DS1. I think a lot of people think we are very brave (or stupid) but I'm glad at the moment we have decided to have another.
Could you get a referral for some genetic counselling - if that might help your decision?