DENVER -- A new study suggests that anal sex is on the rise among teens and young adults, particularly among those who engage in unprotected sex.

In the study, released in November, by Bradley Hasbro Children's Research Center in Rhode Island, researchers assessed the sexual behavior of 1,348 adolescents and young adults between the ages of 15 and 21 who had unprotected sex in the previous three months. They found that 16 percent had engaged in heterosexual anal intercourse within the time frame, with condoms being used just 29 percent of the time.

Females who had heterosexual anal sex were more likely to be living with their partners, to have two or more sexual partners and to have previously experienced coerced intercourse.

"The topic of anal intercourse is often considered taboo -- especially when discussed in the context of youth relationships -- even though we know that this behavior is a significant risk factor for HIV and other sexually transmitted infections. It's critical that we recognize that more and more young people are engaging in anal sex so we can open the lines of communications and help them protect their sexual health," said Celia Lescano of Brown University, the Bradley Hasbro study lead author.

"We must teach teen girls and young women how to be assertive in sexual relationships, such as refusing unwanted sexual acts and negotiating for safer sex, whether it's anal or vaginal."

"It really is shocking how many myths young people have about anal sex," said Judy Kuriansky, a Columbia University professor and author of "Sexuality Education: Past Present and Future."

"They don't think you can get a disease from it because you're not having intercourse," she told ABCNews.com.

The Hasbro study, conducted in Atlanta, Miami and Providence, used audio computer-assisted self-interview technology to allow participants to input their responses directly, without having to talk to an interviewer.

The study was published online by the American Journal of Public Health.

"Given the subject matter, it is likely that the numbers reported may actually be an underestimate of the prevalence of these behaviors," said Lescano.

The results of the study worry AIDS researchers who say anal sex can increase the chance of infection. A CDC fact sheet explained the risks of anal sex. First, "the lining of the rectum is thin and may allow the virus to enter the body." It also said "condoms are more likely to break during anal sex than during vaginal sex."

Anecdotal, ABC News reported, young adults have confirmed the rise in anal sex, including the perception that it is safe.

Other teens said anal sex over vaginal is popular because it keeps them "technically" a virgin.

A well-publicized 2005 study that used data from the National Longitudinal Study of Adolescent Health found that teenagers who take "virginity pledges" were more likely to engage in oral or anal sex than non pledging teens and were less likely to use condoms once they became sexually active.

The study was conducted by researchers at Yale and Columbia universities.

The Journal of Adolescent Health reported that rising rates of anal sex among teens is reflected in the general adult population where anal sex rates have doubled since 1995.

"An open dialogue between health care providers and their young patients about anal intercourse is becoming increasingly important, and clinicians should ask about anal sex during discussions about vaginal intercourse and protection – regardless of the patient's gender or reported sexual orientation," said Lescano.