The two studies, done by the Centers for Disease Control and Prevention's National Center for Health Statistics, are based on ongoing in-person surveys of Americans done by the government.
The teen survey included about 1,100 boys and 1,100 girls aged 15 to 19. The percentage of girls aged 15 to 17 who said they had ever had sex fell from 38 percent in 1995 to 30 percent in 2002, the center said.
The survey found that 55 percent of all male teens said they had ever had sex in 1995, and this declined to 46 percent in 2002.
"Just under half of teens have had sex: In 2002, 47 percent, or 4.6 million, female teens had had sex and 46 percent, or 4.7 million, male teens had had sex," the NCHS said in a statement.
Among teens who said they had recent sex in 2002, 83 percent of girls and 91 percent of boys said they used contraception. This is up from up from 61 percent in the 1980s and about 71 percent in 1995.
"There is much good news in these results," said Health and Human Services Secretary Tommy Thompson.
"More teenagers are avoiding or postponing sexual activity, which can lead to sexually transmitted diseases, unwanted pregnancy or emotional and societal responsibilities for which they are not prepared."
A second study of 7,643 women aged 15 to 44 in 2002 showed that 82 percent had used the birth control pill at some time in their lives and about 90 percent had a partner use a condom.
"The condom is the leading method at first intercourse, and use at first intercourse has been rising steadily in the last 20 years," the NCHS said.
"The pill is the leading method among young women, and female sterilization is the leading method among women over 35." The study found that the pill is used by 11.6 million women and female sterilization is used by 10.3 million women.
"These changes in sexual activity and contraceptive use are consistent with the downward trend in teen pregnancy and births over the past decade," the CDC said in a statement.
WASHINGTON, Dec 10 (Reuters)
By Maggie Fox, Health and Science Correspondent