Feeling guilty

I've been feeling guilty. Two of my closest girlfriends are desperately trying to fall pregnant. I got lucky relatively quickly but am uncomfortable being excited around them. Both of them have said they are happy for me but I know them so well I can see and feel their sadness. One of them even admitted she was really upset the next day.
Do I continue to be excited and talk with them about it when they ask me questions or should I keep my answers brief and change topics like I have been doing?
Has anyone got experience on what to do?