Children as young as five should be given sex education lessons, experts have recommended.
The measures are aimed at cutting teenage pregnancies
A report from the Independent Advisory Group on Teenage Pregnancy says it should be part of the statutory curriculum for all key stages.
The first key stage covers children aged five to seven.
The Department for Education and Skills currently recommends primary schools have a sex and relationship education programme, which should ensure children know about puberty and about how a baby is born.
Five-year-olds would not be taught the mechanics of sex under the proposals.
Instead, it could be touched on in discussions about where babies come from, if a child has a new brother or sister.
Personal social and health education, which includes sex education, is not a statutory part of the national curriculum.
Gill Frances of the National Children's Bureau, who is the advisory group's deputy chairman, told BBC News Online: "What is important is that children are able to talk without shame or embarrassment about all the aspects of their life, and have adults around them who can answer the questions that they have.
"If children acquire a brother or a sister, then they are going to ask questions - and the gooseberry bush won't wash.
"So you would tell them 'Mummy and Daddy love each other and wanted to have a baby'."
She added: "If you establish this ability to talk about these things in the very early years, then when the time comes for more detailed discussions about sex later on, the children have a hook on which to hang the new information.
"This should be happening in every school."
But Robert Whelan of the campaign group Family and Youth Concern said: "We are opposed to sex education in primary schools. It is far too young.
"It is a form of child abuse. You are robbing children of their innocence and giving them information they are not ready for.
"You're taking away their childhood."
A spokeswoman for the Department for Education and Skills said the government had no plans to change its policy on sex education in primary schools.
"Schools are expected to consult parents on the policy on sex and relationship education.
"They need to inform parents about what's being taught.
"And parents have a right to withdraw their children from health and relationship education."
Other recommendations from the advisory group's national report include a national advertising campaign to reassure children under 16 their parents will not be told if they seek contraceptive advice.
It also recommends boys and young men should be targeted in efforts to cut teenage pregnancy rates.
The report said many saw sex and relationship education as "irrelevant".
It recommended young people from black and ethnic minorities should also be targeted.
There has been a fall in the number of teenagers becoming pregnant, with a 10% reduction in conceptions amongst under 18s, and an 11% fall amongst under 16s since 1998.