
Alfie Patten's girlfriend Chantelle Steadman, 15, gave birth to Maisie Roxanne at the Eastbourne District General Hospital, East Sussex, four days ago.
The force had looked at the case and decided it was "not in anyone's interests" for a prosecution to take place, a Sussex Police spokeswoman said.
"Sussex Police Child Protection Team were aware of a 14-year-old girl that had become pregnant as the result of a relationship with a 12-year-old boy.
"A joint agency investigation with East Sussex County Council Children's Services has taken place which has considered the needs of both individuals and there will be continued support for these two young people in the future."