Teen birth rates are at an all-time low. We still don’t know exactly why.

Teen births in the United States are at an all-time low.

The teen birth rate is falling the fastest in Hispanic and black communities – two demographics with the highest teen birth rates in the US, according to a new report from the Center for Disease Control and Protection. Since 2006, birth rates among black and Hispanic 15- to 19-year-olds have dropped 44 percent and 51 percent respectively. The birth rate for white teens dropped 35 percent.

According to the report, in some states the black and Hispanic teen birth rate was even six or seven times higher than the birth rates for white teens:

For example, in New Jersey, the teen birth rate among whites (4.8) was well below the national rate for this group (18.0); whereas teen birth rates in this state among blacks (27.4) and Hispanics (31.3) were also lower than the national rates for these groups (blacks: 37.0; Hispanics: 39.8), they were approximately 6–7 fold higher than the rate for whites.

In other states, disparities reflected birth rates for black and Hispanic teens that exceeded national rates for these groups. For example, in Nebraska, the birth rate for white teens (16.2) approximated the national rate, whereas rates for black and Hispanic teens (42.6 and 53.9, respectively) far exceeded the national rate for these groups.

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

Teen births are universally understood to have great social and economic costs. High birth rates are strong contributors to unemployment: Most teen mothers do not receive financial support from the fathers, drop out of high school, and 48 percent live under the poverty line.

In 2010, the CDC reported that teen births cost the country $9.4 billion in taxpayer money. This is not to mention the potential health risks on a young mother.

Teen birth rate has been falling at an unprecedented — and somewhat mysterious — high rate since 2006. Before 2006, much of the decline in teen birth was attributed to the rise in contraceptives. Vox’s Sarah Kliff explained:

High school students’ use of condoms increased from 43 percent in 1991 to 63 percent in 2002. Rates of sexual activity went down, too. The teenagers of the 1990s were having less, and safer, sex — a nearly sure-fire recipe for fewer teens having babies.

But the more recent decline doesn’t have a clear explanation.

“We have a pretty convincing story for what was going on between 1991 up until about 2005 or so,” said John Santelli, a Columbia University professor who has done extensive research on teen birth. “Now, we’re frankly a little stumped trying to explain the recent decline.”

There are a lot of theories, including the rise of IUDs for teens, increased access to sex education, the popularity of the TV show 16 and Pregnant (Kliff goes into all these theories in more detail), but one thing is for certain: Whatever is going on seems to be working, and hopefully it continues to do so.