Telling a young child they are getting a sibling?

I just don't think he'll actually understand getting a sibling ... so I'm not sure really how to tell him/prepare him ...
Update: My son is almost 9 months. I just found out I am pregnant with another. He will be 15 months when I have his sibling. For moms who have children close together ... I'm just wondering how you tell a child this young about getting another sibling. Do they understand exactly what that means? Do they notice... show more My son is almost 9 months. I just found out I am pregnant with another. He will be 15 months when I have his sibling.

For moms who have children close together ...

I'm just wondering how you tell a child this young about getting another sibling. Do they understand exactly what that means? Do they notice you getting bigger? etc ...
Update 2: Brittany - lol, you're right. I guess mental math is failing me at the moment.
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