More than 1 in 5 sexually active teen girls in the United States have used the morning-after pill - a dramatic increase that likely reflects that it's easier now for teens to buy the emergency contraceptive.

A report released Wednesday by Centers for Disease Control and Prevention, a U.S. government health agency shows teen use of the morning-after pill rose steadily from a decade earlier, when it was 1 in 12. Now, all teens can buy it without a prescription.

The report showed little recent change in most other types of birth control used by teen girls who have had sex. Almost all said they they've used condoms at some point, and more than half have used the pill.

The fact that more teen girls bought the morning-after pill after it became more accessible is a sign that "teens, like adults, often are not very good at contraception," said Bill Albert, chief program officer for the National Campaign to Prevent Teen and Unplanned Pregnancy.

"In the battle between sex and sex with contraception, sex often wins," he said.

The morning-after pill contains a higher dose of the female hormone progestin than is in regular birth control pills. It can cut the chances of pregnancy by nearly 90 percent if taken within 72 hours of unprotected sex.

One of the report's other main findings has been the proportion of teens who said they've had sex. Those figures steadily fell from the late 1980s until the early 2000s - a decrease commonly attributed to improved sex education and greater concern by teens about AIDS and other sexually spread diseases.