Colorado’s teen pregnancy rate has been getting some attention recently. But it’s not for the reason you think; it’s actually for the opposite reason.

From 2009 to 2013, Colorado reported a 40% decrease in teenage pregnancies, according to the Colorado Department of Public Health and Environment. Below is a graph that shows the decline:

That seems insane, right? But there’s actually an interesting reason behind it.

In 2008, an anonymous donor (later revealed to be the Susan Thompson Buffett Foundation, named after Warren Buffett’s late wife) gave a $23 gift to be parceled out over five years. The gift was to be used for “long-term contraception” for low-income teens and women. Over 30K intrauterine devices (IUDs) were purchased and implemented. This measure was rolled out in 68 clinics, as part of Colorado’s Family Planning Initiative.

The IUDs were found to be a very significant factor in the state’s teen pregnancy decline. The study released by the Colorado Department of Public Health and Environment showed that “the percentage of young women receiving IUDs and implants quadrupled in participating clinics,” and, in a complementary effect, the women receiving IUDs accounted for 75% of the state’s overall teen birth rate decline.

On a national scale, Colorado rose from having the “29th lowest teen birth rate in the nation to the 19th.” This is significant as seven in 10 teen pregnancies in the state are unplanned.

The program expires this summer, and it’s unclear whether it will be renewed. But the numbers definitely speak for themselves in terms of effectiveness.