I recently finished a very interesting beta version of an e-class where they encourage you very strongly to be forthright about adoption
especially in front of your children.
The idea is that if you aren't forthright about it then the children will think it is something to be ashamed of (if you think about the things we tend to keep secret, I can understand this concern.)
I would say oh, "I've never been pregnant, my son
was adopted, " and leave it at that.
