When to tell toddler that Im expecting?!

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My Jan 2017 girl is going to be a big sister! I just had my first ultrasound today measuring right on track at 7w3d with a healthy hr of 147! This is my rainbow after an early loss in Feb.

For those with multiple, when did you tell your child that you were pregnant? My daughter always says she wants a baby brother or sister so she will be excited I know!