A3. Your first advance Child Tax Credit payments will be based on the children you claimed for the Child Tax Credit on your 2020 tax return (or 2019 tax return, if your 2020 tax return has not been processed as of the payment determination date for any of your monthly advance Child Tax Credit payments).
Later this year, the Child Tax Credit Update Portal (CTC UP) will be updated to allow you to inform us about the qualifying children you will claim on your 2021 tax return so that we can adjust your estimated 2021 Child Tax Credit – and therefore adjust the amount of your monthly advance Child Tax Credit payments.
If you do not receive advance Child Tax Credit payments for a qualifying child you will claim in 2021, you may claim the full amount of your allowable Child Tax Credit for that child when you file your 2021 tax return.
More details will be provided regarding CTC UP through these questions and answers
https://www.irs.gov/credits-deductions/2021-child-tax-credit-and-advance-child-tax-credit-payments-t...
