Teen birth rate in the country has been falling at an astounding rate for the past five years, which some see as great news, but researchers have yet to discover why it's falling, Jezebel reported on Thursday.

The birth rate has fallen 57 percent since 1991, according to data released by the CDC. The rate of decline was particularly rapid between 2007 and 2013; it fell about 38 percent in that time period. Teen births are unplanned 77 percent of the time and teen mothers are much more likely to drop out of school. The drop has also saved taxpayers $12 billion in 2010 alone, according to the CDC.

One reason for the decline may be that condom use amongst teens increased from 43 percent in 1991 to 63 percent in 2002. Rates of sexual activity declined too. Teens nowadays are statistically having less sex, and when they do, it's safer. The AIDS epidemic starting in the early 1990's is thought to have something to do with teens having safer sex, according to Vox.com.

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

The recession of the late 2000's might have something to do with the current decline, researchers believe. Birth rates started to drop around that time and the same trend has shown up in prior economic downturns. Between 2007 and 2013, births to mothers of all ages fell 8.4 percent. 

Even though teens most likely weren't checking their bank accounts and analyzing if they could have a child due to the unplanned nature of their pregnancies, researchers argue that seeing economic struggle around them could lead to more cautious behavior.

Other theories include the availability of the intrauterine device for birth control, pregnancy prevention classes in some cities and the success of MTV's 16 and pregnant.