Finding out that you now have a child with special needs is difficult. I can't imagine having this happen with your first pregnancy, but in many ways it is difficult in a different way to already have children when it happens. When Trenton was born, our oldest son was two. He really did well. He had some issues with stuttering because of the stress we were all going through being away from home for so long, and that stopped once we got home.
A new baby is always going to affect an older child in some way, but more recently I have seen it affect our oldest more. It is something to think about when you see a family with a few children, one of which has special needs. Sometimes parents cannot give their other special children as much attention as they would like. Sometimes we have to drop everything to take care of an important issue with our child with medical needs.
So what can I do? What can you do? All we can do is our best. Give your other special children as much attention as you can. Keep in mind that you need some alone time as well once in a while. Try to help your child understand what is going on and why you can't play right then. They will not always understand, but all you can do is love them and make sure they know that they are special too.
If you are reading this and do not have a child with special needs but you know people who do, then there is something that you can do. Try not to judge the parents. If you see their older child misbehaving and the parent is struggling, be empathetic. If you see that a child seems to need attention but the parent is busy with the child with special need, volunteer to take him or her for a little while. Having people around who are willing to give that extra attention that you cannot give is a blessing. The parent may or may not agree, but in offering, you did the right thing. If a parent does not need the help right then, don't give up. Most likely there will be a time when your help would be needed.