1. World Health Organization, UNiCEF, UNFPA, World Bank (2012) Trends in Maternal Mortality: 1990 to 2010. World Health Organization.
2. UNiCEF, World Health Organization, World Bank, United Nations (2012) Levels and Trends in Child Mortality Report 2012. UNICEF.
3. Twinning across the Developing World;J Smits;Plos One,2011
4. Sri Lankan twin registry;A Sumathipala;Twin Res,2002
5. Twins in Jos Nigeria: a seven-year retrospective study;A Aisien;Med Sci Monit,2000