Up to one in five millennials do not know what sexually transmitted infections (STIs) are, new research suggests.

Some 20 per cent of those aged 18-to-25 admit to either not fully understanding STIs, such as gonorrhoea and chlamydia, or being completely clueless as to what they are, a survey found.

This is despite 30 per cent of young people who get tested for STIs coming back positive, the research adds.

Research also released today suggests less than half of millennials know when male and female fertility declines, despite 90 per cent of young people wanting families one day.

Zoe Bailie, director of brand at the under-25s London-based charity The Mix, which carried out the STI research, said: 'We know young people have a lot to deal with and testing for STIs isn't always the first thing they think about when it comes to their sex life.'

Ms Bailie adds, however, young people should be more aware of their sexual health and how antibiotics can treat certain STIs before they cause issues like infertility and brain damage.

Up to one in five millennials do not know what sexually transmitted infections are (stock)

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One in five are too embarrassed to get tested

Results of the 1,012-people survey, which was conducted as part of the charity's 'Getting Some? Get Tested' campaign, further suggests many millennials believe STI check ups are only necessary if people suffer symptoms, despite many infections being 'silent'.

Some 44 per cent feel there is no need for such check ups due to them only ever having protected sex, while 20 per cent are too embarrassed to visit a sexual-health clinic.

Communication appears to be key with 53 per cent of millennials saying they would get tested if their partner asked them to, however, three quarters of women say they would never query if their other half had the all clear.

This is compared to 50 per cent of men who ask about their partners' STI statuses before getting intimate.

'Pop culture encourages short-term sexual relationships'

Speaking of the results, south-London GP Dr Martin Godfrey, said: 'Being young is all about experimenting and having fun, including when it comes to sex.

'Pop culture may be seen to encourage short-term sexual relationships without talking about protection and STIs, which can put young people at risk.

'With most STIs being symptomless, young people need to get tested to be sure they don’t have an infection and don’t put their partners at risk.

'Getting tested for STIs is about keeping your sex life healthy (so it stays fun) and supporting positive sexual experiences .'

Less than half of millennials know when male and female fertility declines (stock)

Millennials do not know when male and female fertility declines

This comes as research released today by the Victorian Assisted Reproductive Treatment Authority, Melbourne, found only 38 per cent of men and 45 per cent of women know female fertility declines at 35-to-39 years old.

Of the 1,215 students surveyed, only 18.3 per cent of men and 16.9 per cent of women know that male fertility decreases between 45 and 49 years of age.

Lead author Dr Eugénie Prior said: 'Our study shows that university students overwhelming want to be parents one day.

'However, most also have an unrealistic expectation of what they will achieve prior to conception, whether that be in their career or financially.

'We need to educate young people about the limits of fertility and support them to become parents at a point that is ideal biologically, while balanced against the life goals they want to achieve.'

The scientists add millennials may struggle to become parents if they put it off to achieve other goals.