@DoninGA said "The IRS is working on adding ways to update your status for an advance of the child tax credit to be paid monthly in 2021"
Advance payments will not be allowed for expectant children. The child will have to be born and have a SS# for that. You will, most likely, have to wait until 2022, when you file your 2021 tax return to get any of the expanded 2021 child tax credit.
But, no matter how late, in 2021, the child is born, you will get the full $3600.
