Births to younger teens aged 15 to 17 years have declined over the past 20 years, but still account for about a quarter of teen births, or nearly 1,700 births a week, according to a report released Tuesday by the U.S. Centers for Disease Control and Prevention (CDC).

Younger moms are "of particular concern" as they are not yet legally recognized as adults and are at greatest risk for poor medical, social, and economic outcomes, said the report published in the CDC journal Morbidity and Mortality Weekly Report.

"Although we have made significant progress reducing teen pregnancy, far too many teens are still having babies," CDC Director Tom Frieden said in a statement.

The report was based on birth data from the 19912012 National Vital Statistics System and adolescent health behavior data from the 20062010 National Survey of Family Growth.

In 2012, there were 86,423 births to teens aged 15 to 17 years, accounting for 28 percent of all births to teens aged 15 to 19 years, the CDC report said.

The rate of births per 1,000 teens aged 15 to 17 years declined 67 percent, from 51.9 in 1991 to 17 in 2012, it said. Of them, hispanic, non-Hispanic black and American Indian/Alaska Native teens have the higher birth rates.

The majority of younger female teens are not yet sexually active, as 73 percent in this age group reported they had not had sex yet, the report said.

Parents are a particularly strong influence on the sexual behavior of teens, however, nearly a quarter of females in this age group never spoke with their parents or guardians about sex, it said.

In addition, among sexually experienced female teens, over 80 percent had not received any formal sex education before they had sex for the first time and over 90 percent of teens used some form of contraception the last time they had sex, but most of them relied on methods that are among the least effective.

The CDC said becoming a teen mom affects whether the mother finishes high school, goes to college, and the type of job she will get, stressing that more can be done to prevent pregnancies in younger teens.

"Trying to balance the task of childbearing while trying to complete their high school education is a difficult set of circumstances, even with the help of family and others," said Shanna Cox of CDC's Division of Reproductive Health.

"We need to provide young people with the support and opportunities they need to empower themselves," Cox added.