LOS ANGELES - Health experts have some simple advice for reducing the teen birthrate in the United States - make sure teens learn about abstinence and birth control before they start having sex.

LOS ANGELES � Health experts have some simple advice for reducing the teen birthrate in the United States � make sure teens learn about abstinence and birth control before they start having sex.

It sounds obvious, but it�s apparently needed, according to a report released yesterday by researchers from the Centers for Disease Control and Prevention. Among teen girls who were sexually experienced, 83 percent told interviewers that they didn�t get formal sex education until after they�d lost their virginity.

Altogether, 91 percent of girls between the ages of 15 and 17 said they�d taken a formal sex-education class that covered information about birth control or ways to say no to sex (and 61 percent said they�d learned about both). In addition, 76 percent of girls in this age group discussed one or both of these topics with their parents.

But timing is everything. The fact that most sexually active young women didn�t get clued in about abstinence or birth control until after they�d had sex �represents a missed opportunity to introduce medically accurate information,� the researchers wrote.

The study, published online in the CDC�s Morbidity and Mortality Weekly Report, noted that:

� 14.6 percent of 15-year-olds had had sex, including 8 percent who were sexually active in the previous three months.

� 28.5 percent of 16-year-olds had had sex, including 16.5 percent who were sexually active in the previous three months.

� 38.6 percent of 17-year-olds had had sex, including 29.7 percent who were sexually active in the previous three months.

Only 15 percent of these teens used a birth-control method that was deemed at least �moderately� effective the first time they had sex, including the pill, vaginal ring, IUD or hormonal implant. Another 62 percent used a �less effective� method, such as condoms, sponges, the rhythm method or withdrawal. The remaining 23 percent said they didn�t use any type of contraception when they lost their virginity, the researchers reported.

Overall, the teen birthrate continued to decline, according to data from the CDC�s National Vital Statistics System. In 2012, the birthrate hit an all-time low of 29.4 per 1,000 females between 15 and 19. (In 1991, there were 84.1 births for every 1,000 females in that age group.)

The birthrate declined even more among the subset of teens who are not yet legal adults. For every 1,000 15-year-olds, 5.4 babies were born in 2012 (down from 17.9 in 1991). Among 16-year-olds, the birthrate was 12.9 in 2012 (down from 36.9 in 1991), and for 17-year-olds it was 23.7 in 2012 (down from 60.6 in 1991).

Birthrates among these teens varied significantly by state and by race and ethnic group, the CDC researchers found. The District of Columbia had the highest rate, at 29 births per 1,000 women 15 to 17. At the other end of the spectrum was New Hampshire, with 6.2 births per 1,000.

Nationwide, Latina teens had the highest birthrate, at 25.5 births per 1,000 women 15 to 17. Asian-American teens had the lowest birthrate in this age group, at 4.1. In the middle were whites (8.4), Native Americans (17) and African-Americans (21.9), according to the report.

The study did not include data on births to girls younger than 15. It also excluded information on miscarriages, abortions and stillbirths, since recent figures for those were not available.

Public-health experts are especially concerned about births to younger teens because these mothers �are at greatest risk for poor medical, social, and economic outcomes,� the researchers noted. Among other challenges, new mothers in this age group are significantly less likely to finish high school than are teens who gave birth at age 18 or 19.

About 1 in 4 teen births is to a girl between 15 and 17, according to the CDC. Every week, nearly 1,700 babies are born to mothers in this age group.