Did your 16 year old turn 17 in 2020? They have to be under 17 on Dec. 31, 2020. So you lost a dependent and added a new one. Your stimulus check is based on the number of qualifying people listed on your tax return for 2020. You don't receive a check for the new baby necessarily, it is just based on the number of qualifying people listed on your return.
So, if you lost one qualifying person because they turned 17 and replaced that person with a new baby, you still just have one qualifying person.
Hope that makes sense.
