A store clerk said that to me once when I was pregnant with my fifth child. I told her that of course we know what causes it. It's just that we enjoy it so much, we can't stop! That embarrassed her so much it shut her up. I just laughed.
I tell them "You have got to pick one or two to kill, since you think I have too many. We can solve the problem right now. Pick at least one"
Most of the time they are too stunned to even stutter a word.
Shuts them up real quick
BTW, currently have two left at home.