Twin births in Singapore: a population-based study using the National Birth Registry. 
Twin studies are a most effective method to analyse gene and environment interactions.  Using data from the Singapore National Registry of Births and Deaths (SNRBD), this paper describes the number of twin and multiple births among different ethnic populations in Singapore.  All births recorded in the SNRBD from 1 January 1986 to 31 December 2001 were analysed.  Outcomes measured were twin and triple birth rates (per 1000 maternities) of the 3 main ethnic groups in Singapore (Chinese, Malays and Asian Indians).  Further outcomes were calculated using Weinberg's differential rule to estimate the number of monozygotic and dizygotic twins.  Overall twin birth rates have steadily increased across all ethnic groups (7 to 9/1000).  The largest increase in multiple births among the ethnic groups were twins born to Asian Indian fathers (6.9 to 9.9/1000) and Malay mothers (5.9 to 9.8/1000).  A significant difference in birth rates between the ethnic groups was found during the years 1994 to 1997, where Chinese parents had the lowest multiple birth rates and Asian Indians the highest.  Estimation and ratios of monozygotic and dizygotic twin births differed among the ethnic groups: Asian Indians had the highest ratios, followed by the Chinese and, lastly, the Malays.  The SNRBD has provided an overview of multiple births in Singapore, although the establishment of a national twin register would enable more detailed analysis of genetic and environmental effects in multiple births.