Since 1986, researchers found, the percentage of 15- to 19-year-olds using contraception has increased substantially in many developing nations in Asia, Africa and Latin America.
By 2006, rates varied widely among countries, but on average, about one-quarter of young women said they had used some birth-control method by the age of 19.
However, compared with older women, teenagers' rate of contraceptive failure was 25% higher, and they were more likely to stop using their chosen birth-control method at some point.
Some of the highest contraception failure rates were seen in countries where teens often chose "traditional" methods, like periodic abstinence or withdrawal, the researchers report in the journal International Perspectives on Sexual and Reproductive Health.
That included Bolivia, where the average rate of unplanned pregnancy was 19%, and Jordan, Turkey and the Philippines, where rates ranged from 10% to 14%.
Besides being more likely to use less-effective means of birth control, teenagers in developing countries - like their counterparts in Western countries - likely have a harder time using birth control consistently than adults do, according to the researchers, led by Ann K. Blanc of EngenderHealth in New York.
Teenage girls may feel afraid or embarrassed to seek out more-modern forms of birth control, be unable to afford them, or lack the knowledge of how to use them properly or deal with side effects if they arise, the researchers point out.
As more and more young women in developing countries put off marriage or want smaller families, the need for effective birth control will only grow, Blanc and her colleagues write.
That, they add, means that reproductive-health services will have to keep up.
"An expanded demand for contraceptive supplies, services and information can be expected to challenge the preparedness, capacity and resources of existing family planning programs and providers," the researchers write.
"This trend of expanded demand is likely to be profound, both for the rising number of female contraceptive users and the public sector programs charged with providing family planning services."
Is unplanned pregnancy the result of lack of knowledge or lust?