How do you react?
I'm not confrontational so I keep my mouth shut, but if I have the opportunity to say that they are IVF babies, I will.
I do cringe when I hear it. They have no idea how that question can be offensive to IVF mommas. I think the better way is to ask if twins run in the family.
Though, I think using the word 'natural' in parenting or pregnancy is bad, too.