We were pretty open about discussing it when my son was 3 and little sister was en route. He just kind of accepted the very simple explanations and went about his merry way. The larger thing to prep for was not so much the pregnancy... it was right after arrival. We had to spend a lot of time making sure our oldest realized he wasn't being replaced, even tho most of the attention was on the new arrival.
We are having to deal a lot more with explaining pregnancy now that he is 8 and daughter is 5. One of our close friends is due next month and the questions from our two are much more pointed now.
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