Parents should be forced by law to teach their children about sex, teachers are expected to argue.
Teen pregnancy rates are still too high say experts
Too many teenage girls are becoming pregnant because of lack of knowledge, the Professional Association of Teachers' annual conference will hear.
Tony Reynolds, a teacher from Cambridge, said many parents did not deal seriously enough with children's sex education.
The UK has one of the highest rates of teenage pregnancy in western Europe.
'Taking the blame'
Mr Reynolds, from Over Community Primary School, said a family in Derby, where three sisters aged 12, 14 and 16 were mothers, showed the seriousness of the situation.
Their own mother, Julie Atkins, blamed their pregnancies on schools for not teaching children enough about sex, he added.
Mr Reynolds said: "We don't need to go into the rights and wrongs of this case as to me it is clear that schools cannot and should not be left in a position where they may take the blame for the current situation.
"The delivery of sex education has to be the joint responsibility of both the home and the school.
"Of course, there will be too many parents unwilling or unable to do this at present.
"Therefore the government must ensure via legislation that parents have their responsibilities clearly set out."
Peer pressure meant many pupils were afraid to ask questions about sex when it is covered in class.
Meanwhile, many parents were too embarrassed to tackle the subject at home, he said.
The conference is debating Mr Reynolds's motion demanding new laws requiring parents "to take more responsibility for teaching their children about sex and morality".