July 16 (HealthDay News) -- Birth rates among U.S. teens increased in 2006 and 2007, following large declines from 1991 to 2005, according to a new U.S. government study.

It found that previously improving trends in teens' and young adults' sexual and reproductive health have flattened or may be worsening in some cases.

U.S. Centers for Disease Control and Prevention researchers analyzed national data from 2002-2007. Among their findings:

• About one-third of adolescents hadn't received instruction on methods of birth control before age 18.
• In 2004, there were about 745,000 pregnancies among females younger than age 20. This included an estimated 16,000 pregnancies among girls aged 10 to 14.
• Syphilis cases among young people aged 15 to 24 have increased in both males and females in recent years.
• In 2006, about one million young people aged 10 to 24 were reported to have chlamydia, gonorrhea or syphilis. Nearly one-quarter of females aged 15 to 19, and 45 percent of females aged 20 to 24 had a human papillomavirus (HPV) infection during 2003-2004.
• From 1997 to 2006, rates of AIDS cases among males aged 15 to 24 increased.
• In 2006, the majority of new diagnoses of HIV infection among young people occurred among males and those aged 20 to 24.
• From 2004 to 2006, about 100,000 females aged 10 to 24 visited a hospital emergency department for nonfatal sexual assault, including 30,000 females aged 10 to 14.

"This report identifies a number of concerns regarding the sexual and reproductive health of our nation's young people," Janet Collins, director of CDC's National Center for Chronic Disease Prevention and Health Promotion, said in a news release.

"It is disheartening that after years of improvement with respect to teen pregnancy and sexually transmitted diseases, we now see signs that progress is stalling and many of these trends are going in the wrong direction," she said.

The study also identified a number of racial/ethnic disparities in the sexual and reproductive health of young Americans. For example, Hispanic teens aged 15 to 19 are much more likely to become pregnant (132.8 births per 1,000 females) than non-Hispanic blacks (128 per 1,000) and non-Hispanic whites (45.2 per 1,000). The study also found that non-Hispanic black youth in all age groups have the highest rates of new HIV and AIDS diagnoses.

The study appears in the July 17 issue of the Morbidity and Mortality Weekly Report, published by the CDC.

SOURCE: U.S. Centers for Disease Control and Prevention, news release, July 16, 2009