On a federal tax return, a child born between 01/01/2025 and 12/31/2025 can be entered as a dependent on a 2025 tax return.
No, they are not a dependent for federal purposes until they are born, in 2025.
However, there are a few states that have credits for unborn children, such as Georgia (see: Life Act Guidance) so check your specific's state rules.
No. The dependent must be born in the 2024 year to be a dependent in 2024.