Hi hun,
You can drive yourself crazy googling and reading papers on this, I have done it myself. There are so many theories, you will find stories of people in all sorts of situations who question whether that is what caused their child's autism. I know of some that have detoxed and had metal anaylsis done before a pregnancy to those who just went with it naturally.
I personally believe that some children are genetically predisposed be that due to our environment, immunisations, diet or combination. I do know it is on the increase and that is not just due to better diagnosis.
I have 3 children, my eldest has asd my middle boy is not great socially with others but has no asd symptoms aside and at the moment I am not concerned with my 9 month old daughter, but am watching her like a hawk. I have chosen not have have my 2nd 2 immunised, not because I am against it (I am not) but because I cannot afford the risk of it being a trigger, no one can tell me 100% it isn't.
There are no asd relatives on my side, and on dh side ds has 15 cousins and our son is the only one affected.
I know it's hard, I have said before most people with neurotypical children just watch them grow and develop and delight, but when you have had an asd child that is something you dream of.
If you plan another preg. eat healthy and fresh, take a good quality supplement, avoid dental work with metals, don't drink or smoke, basically everything your dr or midwife would suggest.
Good luck hun, I know its not easy. x