1. Martin JA Hamilton , BE Sutton PD et al. Births final data 2002 2003
2. Annual summary of vital statistics: 2006;Martin;Pediatrics,2008
3. Trends in the occurrence, determinants, and consequences of multiple births;Blondel;Semin Perinatol,2002
4. Prenatal diagnosis in twin gestations: a comparison between second trimester amniocentesis and first trimester chorionic villus sampling;Wapner;Obstet Gynecol,1993
5. Invasive prenatal testing for aneuploidy;ACOG;Obstet Gynecol,2007