Tomorrow's potential troublemakers can be identified even before they are born, Tony Blair has suggested.
Mr Blair wants to stop tomorrow's troublemakers emerging
Mr Blair said it was possible to spot the families whose circumstances made it likely their children would grow up to be a "menace to society".
He said teenage mums and problem families could be forced to take help to head off difficulties.
He said the government had to intervene much earlier to prevent problems developing when children were older.
There could be sanctions for parents who refused to take advice, he said.
The PM has returned from his summer holiday to face intense speculation about his plans to step down.
In his first interview since his three-week stay in the Caribbean, Mr Blair insisted his policies could outlast his time in Downing Street.
His aides say people are more interested in problems like anti-social behaviour than in talk about when the prime minister will quit.
The Conservatives say the government should not try to run people's lives.
And one think tank suggested it was almost "genetic determinism" to suggest children could turn out to be troublemakers before they were born.
Mr Blair told BBC News his government had made "massive progress" in tackling social exclusion but there was a group of people with multiple problems.
There had to be intervention "pre-birth even", he said.
Families with drug and alcohol problems were being identified too late, said Mr Blair.
And there was a "pretty good chance" children of teenage mothers who were not in stable relationships would grow up in a "difficult set of circumstances" and develop behavioural problems.
He admitted many people might be uneasy with the idea of intervening in people's family life but said there was no point "pussy-footing".
But he said: "If we are not prepared to predict and intervene far more early then there are children who are growing up - in families which we know are dysfunctional - and the kids a few years down the line are going to be a menace to society and actually a threat to themselves."
Help had to be offered, but "some sense of discipline and responsibility" had to be brought to bear, he said.
Official figures released in February showed the conception rate for girls aged 13-15 was 7.5 per 1,000 in 2004.
Mr Blair did not specify exactly what changes he was preparing to make or how they would work.
He pointed to the success of Sure Start centres in giving parents help.
And he said sanctions, such as anti-social behaviour orders or parenting orders, were already in place for those who refused support.
But experience from other countries suggested people were willing to get support if they knew where to get it, he added.
Conservative policy director Oliver Letwin said: "The answer is not more state intervention.
"It is to encourage the social enterprise, the voluntary sector, community groups, to help people without trying to run their lives for them."
Norman Lamb, chief of staff to Liberal Democrat leader Sir Menzies Campbell, said: ""Empty threats to pregnant mothers will do little to restore confidence in a government that has failed to tackle poverty, crime and social exclusion for the last nine years."
Anastasia de Waal, head of the family and education unit at think tank Civitas, said: "It is teetering on genetic determinism this kind of saying that before children are even born they are labelled as problematic."
Ministers should not take a "Big Brother" approach to the problem and look at the root causes of problem behaviour.
But Dorit Braun, Chief Executive of national parent support charity Parentline Plus, said he was pleased Mr Blair was talking about intervening before families hit crisis point.
"The difficulties will lie in how this is received by families," she said, saying intervention must be "respectful".