3Cande V,Ananth KS,Joseph,et al.Trends in preterm birth and perinatal mortality Among singletons:United States 1989 through 2000.Obstet & Gynaecol,2005,105(5):1084~1091.
4Drazancic A.Antenatal care in developing countries-what shoud be done? J Perinat Med,2001,29(3):188.
5Jean-Marie M.Classification and heterogeneity of preterm birth.BJOG,2003,110(sup20):30~33.