The East Flanders Prospective Twin Survey, started in 1964, realised for the first time a population-based register of all multiple births in the Belgian Province of East Flanders, characterized by the systematic examination of the placenta and the accurate determination of zygosity based on blood groups, DNA and structure of fetal membranes. For decades, it has been the only register to include chorionicity in same-sex pairs of twins, an irreplaceable tool allowing almost certain diagnosis of monozygosity and the timing of the division of the zygote.in monozygotic twins. We showed that twins and triplets are far from being an homogenised group and demonstrated differences between these groups important enough to introduce these variables in the standard methodology of twin research. Examples are: Weinberg rule, sex ratio and duration of gestation.