Baby Maisie was born at Eastbourne hospital earlier this week
Social services have pledged to support a 13-year-old reported to be the father of a baby born to a girl aged 15.
The Sun newspaper reported that Alfie Patten, of Eastbourne, fathered the baby girl at the age of 12.
His girlfriend, Chantelle Steadman, gave birth to a daughter earlier this week. Sussex Police have said they will not prosecute either of the teenagers.
East Sussex County Council's children's services said there would be "intensive monitoring" of the three children.
Maisie Roxanne was born at the Eastbourne District General Hospital, East Sussex, it was reported.
Any birth to parents this young is a cause of great concern to us
East Sussex County Council
A Sussex Police spokeswoman said the force had looked at the case and decided it was "not in anyone's interests" for a prosecution for sexual activity with a child 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."
Matt Dunkley, director of children's services at East Sussex County Council, said they had worked closely with both families since the pregnancy came to light.
He added: "Any birth to parents this young is a cause of great concern to us and in these circumstances we will always offer substantial support to the families involved.
"We have also worked with colleagues in the health service and with the schools concerned to assess the needs of the young people to ensure they get the right support, and also to ensure the needs of the unborn child would be properly met.
"That process will now continue with intensive monitoring and health visitor support."
The county had just begun a campaign to help young people "take responsibility for their actions and understand the serious consequences of sexual relationships", he added.
Nigel Waterson, Tory MP for Eastbourne, Willingdon and East Dean described it as a "very sad story" that will have a "huge impact" on the parents and the child.
"I'm very pleased that the families are being supportive, but this of course raises huge questions about sex education rather than relationship education in schools, and also about the sexualisation of our society," he said.