Alfie Patten's girlfriend Chantelle Steadman, 15, gave birth to Maisie Roxanne at the Eastbourne District General Hospital, East Sussex. A Sussex Police spokeswoman said the force had looked at the case and decided it was "not in anyone's interests" for a prosecution to take place.
In a statement, police 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."
Thirteen-year-old Alfie is the latest in a long line of young UK teenagers to become parents.
The news that 4ft Alfie conceived baby Maisie when he was only 12 renewed calls for better sex education for the nation's children.
England has long had the highest rate of teenage pregnancies in western Europe.
Jenny Teague, of Dorset, was for many years regarded as the UK's youngest mum giving birth in 1997, aged just 12 to Sasha.
Zoe Hodgson, of Birstall, West Yorkshire, was 12 when she conceived and 13 when she gave birth in 2005 to son Leo, whom she used as inspiration to continue her studies, passing 11 GCSEs.
Edinburgh girl Samantha Robertson conceived her daughter Leigha at 14.
After giving birth and finding life difficult she launched "Sassy Mums" - a website for other young mothers.
Natalie Scanlan, 16, of Stalybridge, Greater Manchester, made national news in 2005 by giving birth to triplets. She went on to feature in a television programme about the nation's youngest parents.
