FRIDAY, June 9 (HealthDay News) -- Many teenage girls feel they are being pressured into having sex, a new U.S. study suggests.

"Unwanted sex occurs often within the sexual relationships of teens," Dr. Margaret J. Blythe and her colleagues at the Indiana University Medical Center, Indianapolis, said in a prepared statement.

In the study, 279 teenage girls between the ages of 14 and 17 were interviewed about their relationships and asked a series of questions -- including questions about whether or not girls felt their boyfriends would become angry or would break up with them if they did not have sex with them.

Of all of the girls interviewed, almost 41 percent said that, on at least one occasion, they had sex when they didn't want to. Ten percent said that their boyfriends forced them to have sex.

Almost 38 percent of those girls also admitted to having unwanted sex because they were afraid that their boyfriend would be angry if they said no. Girls were also more likely to be pressured into sex if they were in a long-term relationship, if they had had a baby with their partner, or if they felt they didn't have control with a partner when it came to sex. Infrequent use of condoms, and the use of marijuana or alcohol, also made a girl more likely to be pressured into sex.

"These unwanted sexual experiences result in risk for sexually transmitted infections and pregnancies. Sexual health counseling to reduce risk should focus on both the patient's and the partner's behaviors," Blythe said.

The findings appear in the June issue of the Archives of Pediatrics & Adolescent Medicine.

Unwanted sex -- including rape and being pressured into sex -- has been related to depression and anxiety disorders in teenage girls, the researchers said. Girls in both long-term and short-term relationships have experienced these problems as a result of unwanted sex.

More information

The National Campaign to Prevent Teen Pregnancy has more information on teen pregnancy.