Global, regional and national levels and trends of preterm birth rates for 1990 to 2014: protocol for development of World Health Organization estimates.
Vogel JP, Chawanpaiboon S, Watananirun K, Lumbiganon P, Petzold M, Moller AB, Thinkhamrop J, Laopaiboon M, Seuc AH, Hogan D, Tunçalp O, Allanson E, Betrán AP, Bonet M, Oladapo OT, Gülmezoglu AM