State-funded children's centres must do much more to help teenage parents and single mothers, a report says.
The government wants 3,500 centres by 2010
The National Audit Office (NAO) found that fewer than a third of the 200 Sure Start centres it investigated were making efforts to reach the most needy.
It also said while families valued the scheme, it was hard to measure whether the £3.2bn investment was good value.
So far, 1,000 centres have opened in England to bring education, childcare, employment and health under one roof.
The scheme was launched in 1998 and is primarily targeted at the under-fives living in deprived areas.
To compile the report, auditors visited 30 centres and interviewed managers at almost 200 of them.
They found more must be done to make baby weighing, health checks and careers advice available to the families who needed most help.
These included lone and teenage parents, disabled children's parents and some ethnic minority families in areas with small minority populations.
The head of the NAO, Sir John Bourn, said: "Though it is too early to tell the long-term impact of Sure Start children's centres on children's lives, we do know that families value the services they provide.
"It is vital that the services reach the most needy members of our communities."
The chair of the Commons public accounts committee, Edward Leigh, called for better information on the cost-effectiveness of the scheme.
"There is as yet a glaring lack of information on the cost-effectiveness of the centres and the services they provide.
"With the numbers of centres increasing so quickly it is vital to know exactly what services we are getting for the taxpayers' money."
Ministers want to open a further 2,500 centres by 2010.
Children's Minister Beverley Hughes said she welcomed the NAO's positive findings and said it helped to focus on the government's own findings of what more needed to be done.
She said she had made it clear to children' centres and local authorities what they were expected to provide for children and parents.
"This included delivering services that are not only of a high quality but are accessible and responsive to the most vulnerable families," she said.
"It also included the steps they must take to manage their performance effectively and demonstrate that they are providing value for money."