More than half of the American teens are having sex before they turn 18. However, contraceptive use has also significantly increased over the years, says a new study by the National Survey of Family Growth, which is administered by the United States Centers for Disease Control and Prevention (CDS).

The study found that 44 percent of males aged 15 to 19 had sex during the period 2011-2015, down from 60 percent in 1988. Among females, the number was down from 51 percent in 1988 to 42 percent in the recent period.

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Most of the 55 percent of teens who have had sex by the age of 18 used some type of protection, typically a condom, the study said.

The increase in the use of protection among teenagers has helped lower the birthrate to 22 per 1,000 females in 2015 from 62 per 1,000 in 1991.

Teen pregnancy rates were at an all-time high in 1990 but with the rising awareness have since fallen more than 50 percent, said Joyce Abma, a researcher at the National Center for Health Statistics, who co-authored the report with Gladys Martinez.

"Teen sexual activity and contraceptive use are the direct mechanisms that lead to teen pregnancy" and sexually transmitted infections, Abma said. "So knowing how prevalent, how common, those behaviors are and how they differ according to different subgroups, demographically, helps policy makers and practitioners know where and how to apply intervention," she added.

The study found that among the 42 percent female teens who had sex, 74 percent had intercourse for the first time with someone with whom they were "going steady," compared with 51 percent of the males.

Twenty percent of the females and nearly 40 percent of the males said their first experiences were with someone they considered "just friends" or "going out (with) every once in a while."

The study also reveals that about 2 percent of females and 7 percent of males said they had sex for the first time with someone they just met.

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The latest numbers released in the report Thursday involved information gathered in interviews with 4,134 teens from 2011 to 2015.

The CDC has been tracking sexual activities and behaviors of U.S. teens aged 15 to 19 since 1988.

“Monitoring sexual activity and contraceptive use among teenagers is important because of the health, economic, and social costs of pregnancy and childbearing among the teen population, the official CDS website says.

Aligned with these results, the rate of teen pregnancy and births in the U.S. has been steadily decreasing since the early 1990s. In 2015, a historic low of 22.3 births per 1,000 teens was recorded.

The CDC survey is conducted annually and involves face-to-face interviews at the participants' homes. The report claims that the teens' responses are gathered in complete privacy. The questions, however, have not changed with the times since the survey began in 1988.