More than half of American teens have had sex by age 18, but teenage pregnancy and birth rates extended their 2-1/2-decade decline because of increased contraceptive use, according to a U.S. government study released on Thursday.

Most of the 55 percent of teens who have had sex by 18 used some type of protection, typically a condom, the study of more than 4,000 teenagers by the U.S. Centers for Disease Control and Prevention's National Center for Health Statistics showed.


Some 80 percent of teenagers employed a contraceptive method during their first sexual encounters, according to the study.

The study measured sexual activity, defined as vaginal intercourse between a female and a male, by teens aged 15 to 19 from 2011 to 2015.

The greater use of protection helped lower the rate of births by teenagers to 22 per 1,000 females in 2015 from 62 per 1,000 in 1991.

Teen pregnancy rates peaked in 1990 and have since fallen more than 50 percent, said Joyce Abma, researcher at the National Center for Health Statistics, who co-authored the report with Gladys Martinez.

In a phone interview, Abma said the level of sexual activity among teenagers fell sharply until about 2002 and had since gradually declined, while the use of contraceptives had steadily increased.

The study found that among males aged 15 to 19, about 44 percent have had sex, down from 60 percent in 1988. For females, that rate was 42 percent in the recent study compared with 51 percent in 1988.


Teenagers are generally more responsible than most parents think when it comes to when and if to have sex, said Bill Albert, spokesman for the nonprofit National Campaign to Prevent Teen and Unplanned Pregnancy.

"Adults, when they think of teen sexual culture, they see little more than a blur of bare midriffs," Albert said.

Among the teen females who have 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, the study found.

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."

About 2 percent of females and 7 percent of males said they had sex for the first time with someone they just met, the study showed.