(CNN) The comments, as soon as they made the rounds, were met with swift condemnation: Rapper T.I. says he takes his daughter to the gynecologist after each birthday to determine if she's a virgin.

As doctors and scientists will tell you, there is no test or exam that can reliably and accurately determine whether a girl or a woman has had sex -- and consequently assess whether she's a "virgin." The very idea of such a test is sexist

And yet, according to the UN, the practice of so-called "virginity testing" has been documented in at least 20 countries around the world

Among them, the United States.

A recently published investigation by Marie Claire and the Fuller Project found physicians in the US still receive requests to perform virginity tests.

And there are no clear guidelines from major US medical organizations on how doctors should address it -- perhaps because, as the American College of Obstetricians and Gynecologists implies, it's a procedure that's not medically valid.

"Anybody who perpetuates the idea that virginity has anything biological to do with it is incorrect and they should be told that," Jennifer Gunter, a gynecologist who writes about women's health issues, told CNN.

"Virginity is a patriarchal, social construct. So keep medicine out of it."

It's a test based on myths and outdated ideas

The so-called "virginity test" is typically performed one of two ways:

Inspecting the hymen for tears or looking at its size and shape.

Through the "two-finger test," which involves inserting fingers into the vagina.

But the practice, doctors say, is based on a misunderstanding of the female body and outdated notions of "purity."

"There's this idea that the hymen is this barrier that remains intact until some Prince Charming comes and 'opens it' and 'pops the cherry,'" said Ranit Mishori, senior medical adviser at Physicians for Human Rights and professor of family medicine at Georgetown University. "All of these horrendous ways of addressing the hymen are patently wrong."

Though people often think the hymen completely covers the vaginal opening, in most cases, it simply surrounds it, Gunter said. During sexual intercourse, it can sometimes be torn or stretched. But it can also tear or stretch through tampon use, sporting activities or medical procedures.

So, it's extremely difficult, doctors say, for them to determine whether any change to the hymen is a result of penetrative sex or other causes.

Another supposed indicator of "virginity" is the presence of blood on the sheets after a woman has sex for the first time That's a myth too.

Studies have refuted the idea that most women bleed the first time they have sex. If they do, it's typically light spotting. What bleeding can indicate, however, is forced penetration or a lack of lubrication, Mishori wrote in a journal article.

It can have harmful consequences

Virginity tests can have damaging psychological consequences for girls and women.

They can cause feelings of guilt, self-disgust, depression, anxiety and a negative body image, several UN agencies said in a statement calling for an end to the practice. In many cases, they're performed at the request of family members or partners, often without consent.

Because there's no equivalent for men, virginity tests imply that sex before marriage is only unacceptable for women.

"If someone's asking for a hymen check, then what they're saying to you is that it's wrong to have sex before marriage," Gunter told CNN.

Sometimes, the consequences of virginity testing are physical.

The sexual stigma perpetuated by the tests can drive women to risky behaviors. They might opt for oral or anal sex to preserve their virginity, doctors say, which can lead to sexually transmitted infections, or STIs, if practiced without protection.

It's based on a narrow definition of sex

People who request virginity tests as evidence of whether a girl or a woman has had sex are missing the point, Mishori said.

There are many other ways to have sex besides vaginal penetration, and simply inspecting the hymen will not yield any instructive answers about whether a woman is sexually active, she said.

Another issue is that the tests don't respect a woman's autonomy and integrity, Mishori added: Why should a woman need to go through an exam to prove something about her sexual history? Why wouldn't society simply believe what she says?

"It's about trusting women, believing women and taking them at face value rather than subjecting them to outsiders being the arbiters of their behaviors, lifestyles or purity or character," she said.

Finally, the idea that someone would even feel the need to assess whether a woman has had sex is a signal of how society determines her worth and desirability, Mishori said.

"There are a lot of social norms that all come into play through that little tiny piece of tissue," she said.

Unfortunately, those social norms and misconceptions have formed over a long period of time, Mishori said, and dismantling them will be a challenge that extends beyond medical professionals.

And if the news this week is any indication, there's still a lot of work to be done.