America's fertility rates just hit an all-time low. Researchers don't know why.

The U.S. fertility rate fell to a record low in 2017, the National Center for Health Statistics reported Thursday, down 3 percent from the year before.

Fertility rates declined sharply after the Great Recession that began in 2008, but researchers are surprised that the rate hasn't bounced back since the economy recovered. Last year there were just 60.2 births per 1,000 women of childbearing age. The drop from 2016 to 2017 constitutes the largest single-year decline since the recession.

There is a growing number of women of childbearing age, but researchers think they are postponing pregnancy to focus on career advancements and achieving financial stability, reports The New York Times. However, the most recent drop showed a decline for women in their 30s, who had previously shown rising fertility rates attributed to the pattern of postponing.

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Fertility rates have dropped even more drastically among minority groups: Between 2007 and 2016, the rate dropped 27 percent among Hispanic women, 11 percent for black women, 5 percent for Asian women, and 4 percent for white women. The good news is the number of teenage pregnancies has declined significantly, dropping a full 70 percent since the rate peaked in 1991. Read more at The New York Times.