A new Yale study has revealed a connection between risky sexual behavior and body mass index.

Research conducted by Yale Center for Interdisciplinary Research on AIDS found that overweight women are at increased risk of contracting sexually transmitted infections compared to women of average weight and obese women. But the results also showed that obese women were significantly less likely to contract STIs than women of normal weight. While all the women included in the study, released Oct. 26, reported a similar behavioral mix of protected and unprotected sex, STI incidence varied among women of different BMIs, suggesting the type of sexual partners the women chose affected the results, said Urania Magriples, an author of the study and an associate professor at the School of Medicine.

The study was funded by the National Institute for Mental Health and focused on young mothers aged 14 to 25. The study included 704 mothers from New Haven and Atlanta, and tracked their sexual behavior for six to 12 months after giving birth. The participants were asked to self-report their condom use and to assess their sexual history during this period.

What distinguished the trial from other studies, Magriples said, were the urine culture tests the women received, which gives the results more credibility.

The study, led by associate medical professor Trace Keshaw, arose from prior research focusing on group prenatal care and Keshaw’s previous work on HIV/AIDS prevention, Magriples said.

“Our previous work shows that obesity and STD risk increase for young mothers,” Keshaw said.

The reason for this increase, however, was unclear, Magriples said.

The previous hypothesis was that the risk of STIs would decline as weight increased because previous studies suggested that overweight and obese women engaged in less sexual activity than normal-weight women, thus lowering the risk of contracting STIs, Keshaw said. But Magriples also noted that it was plausible that overweight women suffering from low self-esteem would engage in more risky behavior.

“We had no idea which way it was gonna go,” Magriples said.

Magriples said the study’s results that although the results show a clear link between weight and STI risk, the results may not apply to a nonurban setting. Magriples also pointed out that the majority of the women studied were Hispanic or African-American, so ethnicity could also affect sexual behavior and perception of weight and esteem.

She added that a limitation of this study and many STI studies is that it is one-sided.

“[There is] not a lot of information [about] the father of the baby,” Magriples said. “[Sexual behavior] is hard to assess when you’re just assessing the woman.”

Some public health experts are skeptical about the practical applications of the study’s results.

Xi Kathy Zhou, assistant professor at Weill Cornell Medical Center, said the results could have been influenced by factors other than weight.

“Association does not mean that obesity led to or caused risky behavior,” Zhou said.

University of California, Berkeley professor Nicholas Jewell said the study is more observational than conclusive. He added that the results are not terribly helpful with regard to prevention unless the mechanism by which weight and STI risk are directly linked is known.

While the results do not definitely link weight with STI incidence, Kershaw hopes that the results will emphasize the importance of targeting multiple health factors.

“We need to create broad programs that help young mothers improve a wide variety of health behaviors including exercise, nutrition and sexual health instead of narrow programs that only focus on obesity or STDs,” he said.

Over one-third of the U.S. population is obese, according to the Centers for Disease Control and Prevention.