The teen birth rate has fallen dramatically in the past few years. Researchers don't fully understand why that is happening (a subject explored in depth here). But thanks to some new federal data released Tuesday, we know a bit more about where the teen birth rate is falling — and which demographic groups have seen the steepest declines.

1) The decline in teen births is massive

We already knew this, but this chart really underscores the point.

The teen birth rate has fallen 57 percent since it peaked in 1991. The past few years have been an especially fast decline, with the teen birth rate plummeting 29 percent between 2007 and 2013 alone.

2) The steepest declines in the teen birth rate are happening in the South

The map above shows birth rates for teenagers between 15 and 17, a group that has had the fastest declines (more on that a bit later).

Among that age group, there are seven states that have had teen birth rate declines over 40 percent. They're geographically clustered in the Southeast and the Southwest.

Perhaps the most surprising part of this map is that there are four states that have no significant change in the birth rate of 15 to 17-year-olds: North Dakota, West Virginia, Vermont and New Hampshire. Vermont and New Hampshire already have really low teen birth rates, which might explain why they didn't see big declines, but North Dakota and West Virginia are a bit of a mystery.

3) But the South still has some of highest teen birth rates in the country

Even though Mississippi, for example, has had really fast declines in its birth rate it still remains one of the states with the most girls having babies.

4) Birth rates are falling faster for younger teens

When the Centers for Disease Control segregated the data into two groups — 15 to 17-year-olds and 18 to 19-year-olds — they saw that, while both demographics were having fewer babies, the decline was steeper for the younger teens.

5. When you slice the data by race, Hispanic and Asian teens have had the steepest declines

While every demographic group the CDC looked at saw declines, the change was especially fast among Hispanic and Asian/Pacific Islander teens. The Hispanic birth rate, for example, fell by 39 percent between 2007 and 2013, a time period when the overall birth rate fell 35 percent.

Head over here to read more about the big decline in teen birth rates.