Teenagers in the United States are having children at a historically low rate, and the decrease in birth rate for black and Hispanic teens has been especially dramatic, according to a report from the Centers for Disease Control.

The teenage birth rate was at a critical level in the early 1990s, but it has now fallen to its lowest level in history. Experts agree that two major factors are in play, one expected and one surprising.

Teenagers today are better able to get contraception than in generations past and that contraception is more convenient, the report said. Injectable and implantable birth control has become more common and can last for years.

The second, unexpected reason the teen birth rate has fallen is that teens are simply having less sex than they were 20 years ago.

“There has been a change in social norms that has happened in the past 20 years, and the idea of not having sex or delaying sex is now something that can be okay,” said Bill Albert, chief program officer for the National Campaign To Prevent Teen and Unplanned Pregnancy, told the Washington Post.

Teenage birth rate in the US was at a peak in 1991, at 61.8 births per 1,000, and has fallen to 24.2 births in 2014, the lowest rate ever recorded. Since 2006, the birth rate of black and Hispanic teenagers has fallen by about 50 per cent, according to the CDC report. Still, minority teens are having more babies than their white counterparts, the report found.

“The United States has made remarkable progress in reducing both teen pregnancy and racial and ethnic differences,“ CDC Director Tom Frieden said, ”but the reality is, too many American teens are still having babies.”