The sharp decline in American teenage pregnancy and birthrates in recent years was driven exclusively by the increased use of contraceptives, a new study concludes.

Researchers interviewed a nationally representative sample of more than 3,000 women ages 15 to 19 at three different time periods: in 2007, 2009 and 2012. They then combined data on sexual activity, contraceptive use and contraceptive failure rates to calculate a Pregnancy Risk Index at these times. This risk index declined steadily at an annual rate of 5.6 percent.

The study, in the Journal of Adolescent Health, found that sexual activity in the last three months of each time period did not vary — about one-third of the young women had had sex during that time. But the percentage of teenagers who reported using contraception increased to 86 percent from 78 percent, and the share using more than one method increased to 37 percent from 26 percent.

The declines in pregnancies and births, the authors conclude, are entirely attributable to improvements in contraceptive use and contraceptive technology.

“These high rates of contraception and low rates of pregnancy are historically unprecedented,” said the lead author, Laura Lindberg, a research scientist with the Guttmacher Institute. “The long-term decline in teen pregnancy and teen births has been brought about by teenagers making considered and responsible decisions.”