Semi-officially, the producers said that the baby was returned to its people in an unseen adventure. I assumed, years ago, that the baby simply died. I mean, they had all those problems with the adult Seven trying to wean her off her Borg nanoprobes and implants, add that to a not-fully-developed infant, and you have a recipe for disaster. She barely survived the episode, who knows what happened next week.
The Borg Baby was one of those aliens from "Unforgetabble" were you don't remember them soon as they are out of your sight for very long. It actually tried to assimilate the ship and was defeated but people soon forgot and was wondering who did all the damage to the ship. Jason
The spies in unforgettable stole the baby to learn about human physiology. No one told them the baby wasn't human. Or They gave the baby up for adoption Or They were on a mission to find the baby a home, something happened, and they had to abort the mission.(sorry)