This is the third annual decline in a row, and 2017 also saw a record low fertility rate of 60.2 births per 1,000 women aged 15-44. The only group to see birth rates rise was women in their early 40s—the birth rate for teenagers was down a whopping 7%, meaning the teen birthrate is now down 55% from 2007.

The obvious problem stemming from the overall decline is that America’s population is aging, which means more older people who may need a social safety net and fewer younger people to pay into the systems that provide it.

What’s also interesting is the mismatch between the declining birth rate and the economy, which is booming. Good economic times usually go hand-in-hand with baby booms.

According to an AP report, experts say the falling birth rate may be due to millennials’ changing attitudes about motherhood—essentially, they’re more inclined to put it off and have fewer kids—as well as increased use of long-term contraception techniques.

“It’s difficult to say yet whether it marks a fundamental change or it’s just a blip, University of Pennsylvania demographer Hans-Peter Kohler told the AP, in reference to the slight decline in birth rates for women in their 30s. That decline marked a reversal of the previous trend.

The new provisional figures come from the Centers for Disease Control and Prevention’s National Center for Health Statistics.

A statistician at the Center, Brady Hamilton, told The Wall Street Journal that he was “absolutely astounded at the continuing decline in teen birthrates.” That decline appears to be at least partly attributable to teens’ increased use of birth control.