when should i tell my toddler about a new sibling on the way?

Why not right away, if he/she is old enough to get the drift? I remember my Mom sitting down to read me a book (as she often did) and it was some type of sex ed for three year olds (undoubtedly Catholic approved) which showed that God had placed a growing baby in Mommy's tummy. I am sure I thought it pretty cool, but I don't remember being excited or even thinking all that much about it.