I never ever ever ask child bearing questions. There are too many different situations that people face in life. You never know if you are talking to someone dealing with fertility issues, someone who had a miscarriage, what have you.
At a birthday party recently, someone asked me if my son would be my last child. I took a breath and said, "Well, my husband died a few months ago so .." I wasn't trying to be mean or anything, but that is why the kitchen is closed. She was mortified, but that wasn't my intent. We talked for a while so she'd know I was okay.
Just goes to show that child bearing questions are really better left unasked.