Hi A.,
First of all, congratulations! I'm a mother of 6, soon to be 7, and we have always told the kids right away. They have always been excited about it. I didn't think the time was so much of an issue, because they get busy and don't think about it all the time. I was probably more impatient than they! But it did allow them to participate in our excitement and preparations, and often even the appointments and ultrasounds, which gave them some "ownership." The only time I wouldn't tell them right away was if I still had some ambivalent feelings about being pregnant, because they will take their cues from you. The most beautiful thing is that I have rarely seen a child who wasn't excited about the prospect of having a baby brother or sister. Even my daughter, who has wanted a sister for so many years, and kept getting brothers, overcame her disappointment and was just as excited as the rest of us. Now, she's finally getting a baby sister in May! They're such a blessing! And your oldest is big enough to really be a helper, too, which will add so much more to the experience for him. Have fun! By the way, if you're a Christian, an awesome kid's book is "Angel in the Waters," by Regina Doman. My kids loved it and wanted me to read it with them all the time, particularly my 4-year-old.