I think "How are you?" was the most hated phrase I heard after I lost our twins, but only when it was asked by folks who didn't know yet. How are you supposed to answer that? "I feel wretched. You see, I just had a miscarriage," is a real conversational bomb to drop on someone just trying to be friendly. I didn't feel right answering, "Fine, and you?", though, because I was definitely not fine, not remotely, and I'm a literal kind of girl. It's amazing how stressful those things you never even thought about before can be when you are grieving, isn't it?
Now, someone who knew the whole story and asked, "How are you?" really helped because they showed they cared; that I wasn't all alone in my grief. God is so good to give us friends who ask after us and just listen. I'm thankful for those questions!
So, what do you do? I suggest ignoring it and saying, "How are you? It's good to see you." You'll know if it is time to share by their response. If they stop and talk and show more interest in your life, by all means share, but an ambiguous response will keep you from dragging your feelings out when you just want to get through the line and check out.