The IRS will update this portal in a few weeks so that you can enter a child born in 2021 for the advance child tax credit - https://www.irs.gov/credits-deductions/advance-child-tax-credit-payments-in-2021

You cannot add a baby that was born in 2021 to your 2020 tax return. The IRS says that in "late summer" the Child Tax Credit Update Portal will have the ability to let you add a child that was born in 2021. If you do not get all the advance payments for your newborn, you will get the additional credit on your 2021 tax return.

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