1. Recommended Definitions, Terminology and Format for Statistical Tables Related to the Perinatal Period and Use of a New Certificate for Cause of Perinatal Deaths: Modifications Recommended by FIGO as Amended October 14,1976;WHO;Acta Obstet gynecoi Scand,1977
2. American College of Obstetric and Gynecology (ACOG). Definitions of Preterm (Premature) Labor and Birth. US,New York; 2016. Available from: https://www.acog.org/Clinical-Guidance-and-Publications/Committee-Opinions/Committee-on-Obstetric-Practice/Definition-of-Term-Pregnancy
3. Preventing preterm births: analysis of trends and potential reductions with interventions in 39 countries with very high human development index;JL Hannah;Lancet,2013
4. The worldwide incidence of preterm birth: a systematic review of maternal mortality and morbidity;DW Stay;Bull World Health Organ,2010