The case of the 13-year-old who has fathered a baby highlights a worrying trend of "children having children", the Tory leader has said.
David Cameron was speaking after Alfie Patten's girlfriend Chantelle Steadman, 15, gave birth on Monday.
Latest figures from UK National Statistics show there were 7,826 conceptions in under-16s in 2006 - down 1,000 from 10 years earlier.
Sexual health experts said teens should be encouraged to have "ambition".
Children's Secretary Ed Balls said it was an "awful" and "unusual" case.
"I want those kids to be safe and also the young child as well, and I want us to do everything we can as a society to make sure we keep teenage pregnancies coming down," he said.
Mr Cameron said: "It is really worrying in our country today you've got children having children - and obviously we all hope that these very, very young children will grow up and be good parents, but frankly parenthood isn't something they should be thinking about.
"That's what's gone wrong and we've got to put it right."