1. Births: preliminary data for 2012a, National vital statistics reports: from the Centers for Disease Control and Prevention, National Center for Health Statistics;Hamilton;National Vital Statistics System,2013
2. Births: Final Data for 2015, National vital statistics reports: from the Centers for Disease Control and Prevention, National Center for Health Statistics;Martin;National Vital Statistics System,2017
3. World health organization. preterm birth fact sheet [online] (mar. 2015), 2015.
4. F.G. Cunningham, K.J. Leveno, S.L. Bloom, J.C. Hauth, D.J. Rouse, C.Y. Spong, Williams Obstetrics 23rd Edition, 2010.
5. The pathophysiology of human premature cervical remodeling resulting in spontaneous preterm birth: Where are we now?;Vink;Semin. Perinatol.,2017