The Centers for Disease Control and Prevention (CDC)
National Center for Health Statistics (NCHS)
Statistics and methodological information on trends for the United States in multiple birth can be accessed quickly from http://www.cdc.gov/nchswww/about/major/natality/natality.htm. For a more complete and current listing of sources, go to http://www.cdc.gov/search.htm and search for "multiple birth" or "triplet birth", or click here and have the search done for you!
The Lancet (requires free registration)
Murphy & Hey, Lancet, 10 May 1997 (requires free registration)
Tong et al., Lancet, 22 March 1997 (requires free registration)
A New Form of Twinning reported by Reuter Information Service in March 1997
Italian surrogate "twins" in The Lancet (1 November 1997) (requires free registration)
The Behavior Genetics Association (BGA)
BGA 1998, Stockholm, Sweden, 8-10 June 1998. You can find program and abstracts from that page.
Ninth International Congress on Twin Studies in Helsinki, Finland, 4-6 June 1998
International Society for Twin Studies
The author of the present article maintains a web page at http://taxa.psyc.missouri.edu/~mbmiller/