Secondary schools must provide lessons in "sex and relationship education", teaching about human reproduction.

But there are opportunities for earlier lessons, with the primary curriculum introducing concepts of life cycles and parents having offspring.

This might mean talking to primary school children about their pets having babies.

But the content and tone of such lessons is still left to the discretion of schools, as policies on sex education are set by school governors, in consultation with parents.

And parents have the right to withdraw their children from sex education lessons that they feel inappropriate.

But there have been clear messages sent by the government on how sex education should be taught - and it appears within the national curriculum.

Guidelines published in July 2000 warned of high rates of teenage pregnancy, and told schools that sex education should "not encourage early sexual experimentation".

This review broadened the term "sex education" to "sex and relationship education", reflecting the inclusion of "moral and emotional" aspects as well as the physical facts of sexuality.

This includes recognising "the importance of marriage for family life; stable and loving relationships, respect, love and care".

The sex education provisions of the Learning and Skills Act 2000, which came into force in November 2000, asserts the duty of schools to be sensitive to the age and culture of pupils and not to introduce "inappropriate" lessons.

The teaching of sex education is most likely to fall in two subject areas - science and in personal, social and health education.

In primary school, this will include teaching children that "humans can produce offspring" and about lifecycles of birth and death.

Five to seven year olds will learn that animals, including humans, grow and reproduce. And that "humans can produce offspring and these grow into adults".

Between seven and eleven, pupils will learn more about life cycles and human reproduction.

Between the ages of 11 and 14, pupils learn that fertilisation in humans is the fusion of a male and female cell.

Students should know the physical and emotional changes that take place during adolescence.

And they are taught about the human reproductive cycle, including the menstrual cycle.

Between the ages of 14 and 16, pupils look at the effect of hormones in humans, including the effects of sex hormones.

They learn how a baby's sex is determined in the womb and examine how hormones are used in medicine - in the contraceptive pill and in fertility treatment, for example.

All maintained secondary schools must provide sex and relationship education for pupils.

This must include education about HIV and AIDS and other sexually transmitted diseases.