1. Births: preliminary data for 2013;Hamilton;Natl Vital Stat Rep,2014
2. Births: final data for 2012;Martin;Natl Vital Stat Rep,2013
3. Births: final data for 2006;Martin;Natl Vital Stat Rep,2009
4. The worldwide incidence of preterm birth: a systematic review of maternal mortality and morbidity;Beck;Bull World Health Org,2010
5. Centers for Disease Control and Prevention. VitalStats. Available at: http://www.cdc.gov/nchs/data_access/vitalstats/VitalStats_Births/htm. Accessed April 11, 2014.