The Centers for Disease Control and Prevention wants doctors to tell teenage boys who are not circumcised to get clipped. The agency is drafting new regulations that would stress to teens that circumcision can help reduce the risk of contracting sexually transmitted diseases from female partners. Dr. Jonathan Mermin, director of the National Center for HIV/AIDS, said circumcision reduced HIV transmission by 50 to 60 percent. Health reasons or not, getting teenagers to go under the knife will be a hard sell. “It’s hard to imagine very many adolescents deciding to have a circumcision done,” said Dr. Douglas S. Diekema of the American Academy of Pediatrics. “It’s a difficult thing to get the adolescent brain to make that kind of calculation.” The CDC is also encouraging expectant parents to circumcise their sons. Although the majority of men in the U.S. are circumcised, the number of uncircumcised men is growing.