A teenage girl’s weight, or perception of her weight, may affect her odds of risky sexual behavior, a new study suggests.

The study, of nearly 7,200 U.S. high school girls, found that in general, sexually active girls who were underweight were less likely than their normal-weight peers to use condoms. The same was true of girls who viewed themselves as overweight - accurately or not - when compared with girls who perceived themselves as normal-weight.

The findings, researchers report in the journal Pediatrics, “add to a growing body of literature that girls at the weight extremes may be at increased risk for engaging in sexual risk-taking behaviors.”

Exactly why this is the case is not fully clear. And the current study found that the question becomes more complicated when race is considered - with weight and perceptions of weight having different effects on white, black and Latina girls’ sexual behavior.

For the study, researchers led by Dr. Aletha Yvette Akers, of the University of Pittsburgh in Pennsylvania, used data from a government health survey that included 7,193 high school girls. The girls were asked about, among other things, their sexual behavior, actual weight and height, and whether they perceived themselves as overweight, too thin or “about right.”

Overall, half of the girls said they had ever had sex, and actual body weight showed no influence over those odds.

Perceptions did seem to matter, however, Akers’ team found. Girls who saw themselves as overweight, however, were about 20 percent less likely to say they were sexually active. At the same time, though, when these girls were sexually active, they were more likely than their peers to have started having sex before age 13.

Across the study group, the only time actual weight was related to sexual behavior was in the case of underweight girls - who were 60 percent less likely than their normal-weight peers to have used a condom the last time they had sex.

However, a closer look showed that many of these general trends varied by race and ethnicity.

For example, the researchers found, among Latina girls, both those who were objectively overweight or underweight were at greater risk of having had sex before age 13 or having had at least four sexual partners, respectively. That was not true of their black or white peers.

Among black girls, those who were actually underweight were less likely than normal-weight girls to use condoms. Meanwhile, those who saw themselves as overweight were at increased risk of having four or more sexual partners in their lives.

For white girls, actual weight showed no effects on sexual behavior. But girls who viewed themselves as overweight or underweight, rightly or wrongly, showed increased odds of certain risky sexual behaviors.

Weight or perceptions of weight could potentially affect sexual behavior for a number of reasons, according to Akers and her colleagues. Studies of adult women, for example, have found that those with a poor body image are less likely to insist on condoms.

“We speculate that girls with a negative body perception may have a limited capacity or willingness to negotiate effectively with partners,” the researchers write, “resulting in higher rates of sexual risk behaviors.”

Whatever the reasons, the findings suggest that the way girls perceive their weight - as well as “cultural differences” in those perceptions - may be as important as actual weight in swaying her sexual behavior, the researchers conclude.

SOURCE: Pediatrics, November 2009.