More than one in 10 people (11 per cent) do not realise a woman can get pregnant if she has sex standing up, according to a poll out today.
The Department of Health and the Department for Children, Schools and Families (DCSF) commissioned the survey to highlight misconceptions around safe sex.
It found 19 per cent were unaware a woman can get pregnant while she is on her period and the same number were unaware she can get pregnant if the man withdraws before ejaculation.
The survey of 2,000 people aged 16 to 50 found 26 per cent never discuss contraception and almost one in three (31 per cent) never talk about sexual health - such as sexually-transmitted infections - with their partner.
A total of 17 per cent are unaware that some infections, such as herpes and genital warts, cannot be cleared up with antibiotics.
Around one in five (21 per cent) people say they chat about sex without actually saying what is on their mind and 62 per cent turn the discussion into a joke.
More than a quarter (27 per cent) admit to being too embarrassed to ask the questions they would really like to while 25 per cent wish they could talk more openly about sex and relationships.
Nevertheless, 77 per cent of people said they would give a friend advice on a sexual health dilemma even if they were unsure of their facts.
Almost half (47 per cent) of people said they never discuss their sex lives with their friends and 37 per cent never talk about contraception.
Sex and relationships expert, Dr Pam Spurr, 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're 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."