Gordon Brown today said he would prefer teenage girls not to get pregnant, claiming the vast majority of the pregnancies were unwanted.
But he appeared unable to say how many young mothers would be affected by his proposal to put them in supervised accommodation if there was not a family member to look after them.
Yesterday in his keynote address Mr Brown unveiled the policy aimed at preventing young parents from automatically getting a council home.
The Prime Minister said it was not right that a 16-year-old girl could "get pregnant, be given the keys to a council flat and be left on her own".
Instead, he told the Labour Party's annual conference in Brighton, groups of young mothers and fathers would be taught responsibility and how to raise their children "properly".
Quizzed about the policy on BBC Radio 4's Today programme, Mr Brown said there were 20,000 teenage pregnancies every year and they were unwanted "in almost every case".
He would prefer a family member to be able to look after a 16 or 17-year-old parent but if that was not the case and they wanted benefits they would go into hostel-style accommodation, which "seems to work".
He said: "We think there is about 20,000 teenage pregnancies every year. They would have to make a choice.
"We would prefer people who are teenagers ... we would prefer them not to become pregnant because these are unwanted pregnancies in almost every case.
"The second thing is we would prefer a mother or a parent or a family to take care of them.
"But if that is not the case and they are coming to ask for taxpayer support, at the moment because of housing need with a young child and being homeless they can get council accommodation in some cases."
Mr Brown added: "About two thirds of those who get accommodation from the Government or local authorities go into council houses. I don't think that is right because they are living on their own and they are living in a way that doesn't change their lives.
"There is a third at the moment who go into hostel accommodation - that seems to work."
But asked how may young women that was, Mr Brown said: "We think at the moment two thirds go into council houses and we will have to make provision for the extra houses but that again is costed in our budget.
"It is cheaper for us in the long run not to have to build a social house at £60,000 and provide accommodation in a centre or a foyer for less than that."