However, it appears teens have not increased their use of highly effective contraceptive methods such as intrauterine devices and they continue to use withdrawal, which is one of the least effective ways to prevent pregnancy.
"One of the goals in this report is to look at factors that influence teen childbearing," said Gladys M. Martinez, statistician at the CDC's National Center for Health Statistics and one of the authors of the report, which was released on Wednesday.
The researchers looked at sexual activity and contraceptive use, which are the direct determinants of whether a person gets pregnant, Martinez said. These factors are influenced in turn by other factors, such as teens' living conditions, which were not examined in the current report.
The study found that 44% of females and 47% of males in the 15- to 19-year-old age group reported having had sex between 2011 and 2013, the years that the teens were surveyed. Those rates have not changed from the previous survey period, 2006 to 2010, although they are much lower than in 1988 when 51% of females and 60% of males in this age group reported having had sex.
The researchers also used the survey data from 2011 to 2013 to determine the likelihood of teens in each age group having had sex, which had not been done in previous reports. They found that 13% and 18% of 15-year-old females and males, respectively, had had sex. Among the 18-year-olds, the rate was up to 68% of females and 69% of males.
Similar to the trend in sexual activity among teens, the rate of contraceptive use does not seem to have changed much, at least between the 2002, 2006 and 2011 to 2013 survey periods. For example, 97% of female teens in 2011 to 2013 reported having used a condom at least once, compared with 94% in 2002.
The researchers also looked at a number of other contraceptive methods, and how many female teens reported ever using one of them. The rates of using the pill or withdrawal have been about 55% to 60% since 2002; use of intrauterine devices was 3% in both 2006 to 2011 and 2011 to 2013.
The exceptions included the use of Depo Provera injectable birth control, which dropped from 21% in 2002 to 15% in 2011 to 2013. The use of emergency contraception increased from 8% in 2002 to 22% in 2011 to 2013, possibly because it is now available to women 15 and older without a prescription.
However, the rates in this report only reveal how often teens have ever used contraception such as a condom, not how consistently they use it, said Laura Lindberg, principle research scientist at the Guttmacher Institute. "What is going to drive the teen pregnancy rate and protection from STDs is using it every time you have sex," she said.
Lindberg suspects that teens are actually using contraception more in recent years because the rate of teen pregnancy has been declining dramatically since the mid-2000s, yet there has not been a steep drop in the rate of teen sex.
Even though the current CDC study reported a 25-year low for teen sexual activity, the time between 1988 and 1995 is really what is responsible for this decline, when there was increased education about sex in response to the HIV epidemic, and the rate of sexual activity has not changed much since then, Lindberg said.
It is not a concern to Lindberg that the rate of sexual activity might not really have been tapering off in the last couple decades "It's normal for teenagers by the time they enter young adulthood to be having sex, so I don't want to problematize that," Lindberg said. "We want the rate of regular contraceptive use to be as high as possible, and we want healthy relationships," she added.
The study found that teens who waited until they were in their late teens (18 and 19) before having sex were more likely to use contraception the first time they had sex, possibly because they were more educated about sex and pregnancy, said Martinez, the study author. And female teens who did not use contraception the first time they had sex were two to five times more likely to have a baby during their teen years.
"First sex is a very key point because it sets you up on a trajectory of what you're likely to do later on," Martinez said.
There can be a number of negative consequences to teenagers having children. "One of the major concerns is that the child has a greater chance of being raised in poverty and the mother's education and employment may be limited [and] the father may be less involved," Lindberg said.
The researchers used data from in-person surveys of more than 2,000 female and male teenagers in households across the U.S. as part of the National Survey of Family Growth.