When I was pregnant with baby #3, my daughter was 5 and son was 2. We shared it with them right away, the 2 yr old didn't quite understand, so therefore, it didn't matter to him. My daughter had so much fun because we read the book "Your Pregnancy Week by Week" together and she loved speculating about what her brother or sister would look like at that point. We even let them take votes on what to name him. We picked the names we liked and had them vote from those. They loved it and felt like a part of the whole thing. She also was just learning to read, so she read to my belly every night and when the baby got here, he recognized her voice right away. My daughter, whose 9 now, still is very close to the youngest one, who is 3 now.