One advertisement, entitled 'Moaning', asks parents how they would cope if their teenage daughter had a baby. A second, which draws an analogy between sport and sex, is called 'Squash' and asks parents whether their sons have sex and know about contraception.
The campaign, launched by the Family Planning Association, is sponsored by the Department of Health and will focus on parental responsibility. Doreen Massey, FPA director, said 20 years of 'debate about sex education rather than sex education' had left thousands of parents 'ill-equipped' to tackle the subject.
She said that, despite the sophistication of teenagers, many still believe 'you can't get pregnant the first time', or 'it is OK if you do it standing up, or after a period'. More than 70,000 teenage girls give birth every year.
The two radio advertisements - described as explicit and hard-hitting by Tom Sackville, Under-Secretary of State for Health, yesterday - will be heard initially in Greater London and the north-west of England, which have the highest rates of under-16 pregnancies.
Speaking at the launch in London, Mr Sackville said: 'I hope this campaign will not be trivialised . . . There are people who say that anything explicit is anti-family. There is nothing more anti-family than a young girl getting pregnant.'
Sex education was a key responsibility of parents, Mr Sackville said. They should not expect schools or doctors to shoulder the burden. We had 'gone too far down the line' of expecting the State or medical profession to teach young people about sex and contraception when it was parents who should be talking to young people, he added.
The UK has one of the highest rates of teenage pregnancies in Europe; under the Health of the Nation initiative, the Government aims to halve the number of conceptions to under-16s in England by 2000, from 9.5 in every 1,000 girls in 1989, to 4.8 per 1,000. Between 1990 and 1991, the rate fell by 8 per cent, from 10.1 to 9.3 per 1,000. The rate must continue to drop by 8 per cent each year for the next six years to meet the government target.
Ms Massey said the FPA recognised the key role parents have in educating their children about growing up, relationships and sexuality. But studies show that, although teenagers want their parents to be their main source of sex education, they receive more information from friends, at school or from magazines.
A free FPA booklet, Answering your child's questions, is available on 0800 555777.