Percentage of infants born less than 37 completed weeks of gestation at birth for any/all causes:
2014: 10.0%
2013: 10.0%
2009: 12.2%
2008: 12.3%
2006: 12.8%

Percentage of infants born weighing less than 5 lb. 8 oz. at birth for any/all causes:
2014: 8.0%
2013: 8.0%
2009: 8.2%
2008: 8.2%

Source

Federal Interagency Forum on Child and Family Statistics. America’s Children in Brief: Key National Indicators of Well-Being, 2016. Washington, DC: US Government Printing Office, 2016.
Federal Interagency Forum on Child and Family Statistics. America’s Children: Key National Indicators of Well-Being, 2011. Washington, DC: U.S. Government Printing Office, 2011.