How do we add a newborn to a pending I-829 petition?

You may be able to make an interfiling with USCIS for new family members, with the assistance of an immigration lawyer. Please remember that children born in the U.S. will be U.S. citizens.

A child under two years of age entering the United States with the mother upon her first reentry to the United States after the birth of the child, is registered automatically at the U.S. port of entry as a lawful permanent resident.

The child can receive a green card if the child enters the United States with his or her permanent resident mother during the mother's first return to the United States after the child's birth and before the child turns two years of age.

Probably the best thing to do is include the newborn upon your transfer from conditional status to permanent status of the green card.

Since the parent of the child is a U.S. resident, it would be very easy for the child to be born in the United States, but if the child is born outside of the United States and is a non-U.S. citizen, since you have already acquired your permanent residency on a conditional basis, the child cannot be included with you. This could be tricky as you would have to wait an additional couple of years and become a U.S. citizen once you meet the five-year residency requirement from the date you acquired your conditional residency, provided you have resided in the United States for half of the applicable five year period and you have never been out continuously more than six months before you could petition for your new child and the child could acquire lawful permanent residency.

You can file a following-to-join processing.

Since you are already a permanent resident, this is the easiest and cheapest way to go. Just present this at the border the first time you come to the United States with the child under two years old.

If the child is born abroad before or after the I-829 is approved, the child is eligible for NA-3 derivative processing. Be careful to make sure the child enters with the mother upon the mother's first entry within two years of the birth of the child. These cases need specialist counsel.