Many people are shaky on the basics of sexual health
One in ten people mistakenly believes it is impossible to get pregnant through having sex standing up, a survey shows.
The Department of Health and Department for Children, Schools and Families commissioned the poll to highlight misconceptions about safe sex.
The poll, of 2,000 people aged 16-50, found 19% were unaware a woman could get pregnant while having a period.
A similar number thought withdrawing before ejaculation was completely safe.
Although teenage pregnancy rates are in decline, with births at their lowest level for more than 15 years, the survey suggests that a combination of bad advice, misinformation and poor quality conversations is hindering further progress.
The survey also found that 26% of respondents never discuss contraception.
One in three said they never talked about sexual health with their partner.
And one in four said they wished they could talk more openly about sex and relationships.
About 85% agreed with the assertion that sex was no longer a taboo subject - but 62% of people admitted to turning discussions on the topic into a joke.
The survey also found a lack of knowledge about sexual health was widespread.
For instance, 17% of respondents were unaware that some sexually transmitted infections - such as herpes and genital warts - cannot be successfully treated with antibiotics.
However, 77% of people said they would give a friend advice on a sexual health dilemma - even if they were unsure of their facts.
Dr Pam Spurr, a sex and relationships expert, said: "In spite of our love of talking about sex and relationships, the survey suggests it's our lack of knowledge that is causing confusion.
"I'm encouraged to see that we've overcome a long-held aversion to talking about sex, but we've still got some way to go before we swap jokes and banter for the open, honest and informed conversations about sexual health and relationships that most of us would like."
Lisa Power, head of policy at the Terrence Higgins Trust, said: "We are delighted that the government has acknowledged the importance of both better education and peer influence in the way people make decisions about their sexual health.
"Ignorance is just as transmissible as chlamydia or HIV and we need to take firm steps to prevent all of those things from spreading."