What would you do? (possible TW)

Let's say you are unable to have your own children and you and your spouse want to be parents more than anything. One day you find an abandoned newborn on the sidewalk. You bring the baby to the authorities and they say there are no missing claims for this baby. Weeks pass and still no one claimed it. You figure it's a sign that that's your baby. So you decide to go forward and legally adopt the baby. You raise the baby for two years and a half years, they know you as mom/dad, and you and your spouse consider that your own child. Then one day you get a knock on the door. It's the biological mother. She says she abandoned her baby bc she was going through a dark time in her life, but is much better now and would like to have custody of her child back. She threatens to take it to court. So what do you do? Fight to keep custody, or grant custody to the bio mom? ... (Btw this isn't real, I got this scenario from an old novela)