We have always told ours as soon as we know, which was always within a few weeks of falling pg. We figured if we had a m/c or complications of any sort, the kids should still be entitled to grieve that loss with us and would otherwise be confused if we were devastated and upset and they didn't know why. And that they should be entitled to feel the joy of knowing life is (all going well) going to get more exciting for them, too!