Africa and Asia make up about 80% of all twin deliveries in the world at present. Prof Christiaan Monden, the study's author from the University of Oxford, said there was a reason for that. "The twinning rate in Africa is so high because of the high number of dizygotic twins - twins born from two separate eggs - born there," he said. "This is most likely to be due to genetic differences between the African population and other populations." Twinning rates in Europe, North America and Oceanic countries have been catching up - and the increasing use of medically assisted reproduction since the 1970s - for example IVF, ICSI, artificial insemination and ovarian stimulation - have been the main reason.
