Should we apply for adoption credit in 2018 or 2019. Baby was adopted Oct 1 2018 and all fees were paid in 2018. Adoption will be final March 4, 2019?

Baby was adopted and placed in our home Oct 1, 2018. All legal and adoption fees were paid in 2018.  Adoption will be final March 4, 2019. We do not know if we can claim adoption credit on 2018 taxes or wait until 2019 taxes.