Updated

Teen births in the U.S. have hit a record low, federal data released Thursday show.

The teen birth rate dropped by 8 percent last year, reaching 22.3 live births for every 1,000 teenage girls, according to the Centers for Disease Control and Prevention. The teen birth rate was 24.2 births per 1,000 teenagers in 2014.

The decline continues a major downward shift in teen births, which have dropped 46 percent in the last eight years. While it's not clear exactly what combination of factors have caused the shift, the decline is encouraging to policymakers and health advocates who have long tried to discourage teen pregnancy.

The 2015 rate is the lowest it's been since the federal government began tracking teen births in 1940. The number of teen births fell to 229,888 last year from 249,078 in 2014, the CDC said.

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