1. 1.World Health Organization. Preterm birth. 2018. https://www.who.int/news-room/fact-sheets/detail/preterm-birth.
2. Epidemiology and causes of preterm birth;Goldenberg;Lancet,2008
3. Trends in preterm birth and perinatal mortality among singletons: United States, 1989 through 2000;Ananth;Obstet Gynecol,2005
4. 4.Martin JA, Hamilton BE, Osterman MJ, Osterman MJ. Births in the United States, 2016. NCHS Data Brief 2017;287:1-8. https://www.cdc.gov/nchs/data/databriefs/db287.pdf
5. Born too soon: the global epidemiology of 15 million preterm births;Blencowe;Reprod Health,2013