Not an expert, but basicly - fertility doctors harvest eggs and sperm, combine them to get fertilized eggs, make a ton more than are needed, because they don't know how many they'll need, how many will survive, then picks a couple and inserts them. The rest are stored in case those first couple don't hold on. If they do, they're tossed - valuable stem cells and all.
I don't think that's the only spot where they are created, but it's a big one, and they're all going to waste. And the stem cells have so incredibly much promise to cure people who today die of terrible wasting diseases, so many different diseases.