Not quite half of American teens have had sex by 18

American teens may be becoming more conservative about sex.

According to a new Centers for Disease Control and Prevention report reflecting data from 2011 to 2015, about 42 percent of girls and about 44 percent of boys ages 15-19 reported that they'd had sex. Both numbers continue a gradual decades-long decline: In 1988, 51 percent of girls and 60 percent of boys in the same age group answered affirmatively on the question of sexual activity.

The latest report, is based on a nationally representative sample of about 4,100 teens. Eighty percent of them used contraception during their first sexual experience, it found.

Previous studies on teens' sexual attitudes and behavior also reveal how today's teens differ from older brothers and sisters as well as parents who grew up in the more permissive 1970s and '80s.

Teen pregnancy and birthrates have been falling since the early 1990s and reached a historic low in 2015. Public health experts credit a combination of widespread access to contraception, plus the decrease in the percentage of teens having sex.