Twin births have major consequences for health and well-being of individuals and health systems. In developed countries twinning has increased tremendously since the 1970s. About trends in twinning rates in developing countries much less is known.

Given the complications surrounding twin births, the major changes currently taking place in twinning rates, and the fragmentary knowledge that is available, it is important to know how many twins are born on earth, where they are born, and how their rates are changing over time. To answer these questions we assembled data from many sources for nearly all countries of the world, and calculated absolute and relative numbers of twin births at national, regional and global level. We produce the world twinning map for 1980 and 2005 and examine changes in between.

Several insights appear from our results: (1) the proportion of twin deliveries is higher than ever before in nearly all countries; (2) the absolute number of twin deliveries is higher than ever before at the world level, and also for all regions of the world; (3) differences between countries in twinning rates have diminished between 1980 and 2005 with a movement of convergence towards high twinning rates nearly everywhere.