P.O.
I personally would not have explained being pregnant to that age until they were able to see the evident signs that you are having a baby, but since that's already done, then I would probably say something like the baby is like a seed that started to grow but it didn't get all it needed and so it stopped growing. End on a good note by saying, in time you will have another one growing again. I think if you mention the human side of it, she might begin to ask questions about dying and so forth and then it can get complicated.