That doesn't mean sex ed never helps prevent pregnancy, researchers said. But attitudes of family and friends, and whether teens have access to contraception and family planning services, might be just as important to putting a dent in the number of new teen moms.

"Although the teen birth rates and teen pregnancy rates are dropping year after year... we still have disparities between states, and we have higher teen birth and teen pregnancy rates when we're compared to other industrialized countries," said Patricia Cavazos-Rehg, from Washington University in St. Louis, who worked on the study.

Researchers said there have been mixed results when it comes to whether a comprehensive sex ed program, which teaches contraceptive use and HIV prevention, does prevent teen births.

Whether school programs should include anything other than abstinence-only education has also been the topic of political battles in some parts of the country.

To get a big-picture take on sex ed's effectiveness, Cavazos-Rehg and her colleagues compared curriculums in 24 states with birth rates for girls ages 15 to 17 in those states between 1997 and 2005.

They found wide variation over the study period, from one birth in every 100 New Hampshire girls each year to three or four births for every 100 Arkansas girls.

In general, the more districts in a state that covered how to use a condom, how to prevent HIV and other sex ed topics, the fewer teen births there were, the researchers report in the Archives of Pediatrics & Adolescent Medicine.

But when they took into account race, poverty and crime levels in states, much of the link disappeared: poorer states with more minorities and more crime had less sex ed and more teen births. And when "religiosity" and laws on abortion were thrown into the mix, sex ed programs themselves no longer predicted birth rates.

Researchers said that there are a number of ways to interpret the findings -- but none of them suggest teens can't learn from sex ed.

First, conservative and religious states might teach sex ed in a less effective way than liberal ones -- meaning more teens in those states end up pregnant.

"There can be enormous variation between what goes on in one state and what goes on in another state even if they both indicate that they discuss how to use a condom or pregnancy prevention," said Amy Bleakley, who studies teen sexual behavior and reproductive health at the University of Pennsylvania in Philadelphia.

Or, it could be that teen pregnancy rates are more similar between states, but in places with liberal abortion laws, more adolescent girls end their pregnancies early.

Cavazos-Rehg said that abortions may account for some of the difference in birth rates, but not all of it. Although data on pregnancy rates are less comprehensive, she said, her own research has still shown much higher teen pregnancy rates in Arkansas and Mississippi, for example, than in New England.

Regardless of what teens are taught in school, whether they can easily get condoms and have their parents' support for "healthy sexual decision-making" may depend on state politics and social factors, Marla Eisenberg, an adolescent health and medicine researcher at the University of Minnesota in Minneapolis, told Reuters Health in an email.

"In short, school-based sex ed is only one part of a much larger puzzle of influences on adolescent sexual behavior, and as the authors conclude, the demographic, religious and political environment matters a lot," added Eisenberg, who didn't participate in the new research.

Still, sex ed can be improved, the experts agreed.

To prevent teen pregnancies, kids in sex ed need to be taught more than just how to use a condom, but the consequences of getting pregnant and having a baby, according to Cavazos-Rehg.

Bleakley, who also wasn't involved in the new study, called for better standards regarding what goes into a comprehensive sex ed program, and better training of instructors across states, to erase any disparities that could be tied to the religious and political environment in a given area.

SOURCE: bit.ly/AquJBE Archives of Pediatrics & Adolescent Medicine, online February 6, 2012.

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