Data collection on multiple births — establishing twin registers and determining zygosity

Twins are a valuable resource not only for studies on multiple births themselves, but on the etiology of diseases and other phenotypes. The method of ascertainment and selection of twins can be crucial for such studies and population based twin registries are thus of great importance as tools of research.Accurate determination of zygosity and chorionicity is essential in all studies of multiple births and in their professional care. The parents and the multiples ask for it. It is of pre-and postnatal medical importance and now considered as a prerequisite in several domains of twin research. It is also important for educational reasons as it helps the multiples and their parents and teachers to ascertain identity. The methods are briefly described and a plea is made to the obstetricians and pediatricians to use them systematically at birth. The distribution of zygosity and chorionicity types among spontaneous and induced twin births are illustrated.