What to tell them?

First off let me tell you how excited I am to see this forum growing! Thanks Roonie! Okay, here is my quandry...our 3 foster siblings have been w/ us since Jan 10th. They have not seen their bmom since Jan 6th if not before. She more or less went missing from 2/2 to 3/20....SW couldn't track her down, all mail returned, all phones disconnected, etc. Well on 3/20 she shows up at SW's office. SW came to our house later that day for her bi-monthly visit and shared with us that bmom is pregnant. She is guessing she's about 6 months along. We are more than likely going to end up with this baby in our home. While this all could become a moot point come the end of April when we go to court again, right now I'm questioning what we should say to the kids to prepare them. If they have not had any visitations with her before the baby is born, they are not going to know she's pregnant unless we tell them. Its highly possible she will run again and we'll never see this baby. However, if she is going to truly try to get these 3 back, she's going to have to stay here! I don't know if we should wait till the baby is born and here before saying anything to the kids. I know if I were pregnant myself, we'd be discussing it all along to prepare them for the huge changes. I also hate waiting till the very very last minute before getting the crib and stuff set back up! However, I don't know what we'd say/do if the baby doesn't come! Plus how do I explain to them that this is really their brother or sister and that mommy J had this baby. (this is assuming they haven't seen her to know she's pregnant). I know I'm probably worrying too much about all this, I just know if it were me that were pregnant, there would be a lot of discussing whats going to happen. Any words of wisdom, please share!! Thanks so much! Shannon