Twin Peaks: more twinning in humans than ever before.

study questionHow many twins are born in human populations and how has this changed over recent decades? SUMMARY ANSWER: Since the 1980s, the global twinning rate has increased by a third, from 9.1 to 12.0 twin deliveries per 1000 deliveries, to about 1.6 million twin pairs each year. WHAT IS KNOWN ALREADY: It was already known that in the 1980s natural twinning rates were low in (East) Asia and South America, at an intermediate level in Europe and North America, and high in many African countries. It was also known that in recent decades, twinning rates have been increasing in the wealthier parts of our world as a result of the rise in medically assisted reproduction (MAR) and delayed childbearing. STUDY DESIGN, SIZE, DURATION: We have brought together all information on national twinning rates available from statistical offices, demographic research institutes, individual survey data and the medical literature for the 1980-1985 and the 2010-2015 periods. PARTICIPANTS/MATERIALS, SETTING,

methodsFor 165 countries, covering over 99% of the global population, we were able to collect or estimate twinning rates for the 2010-2015 period. For 112 countries, we were also able to obtain twinning rates for 1980-1985. MAIN RESULTS AND THE ROLE OF CHANCE: Substantial increases in twinning rates were observed in many countries in Europe, North America and Asia. For 74 out of 112 countries the increase was more than 10%. Africa is still the continent with highest twinning rates, but Europe, North America and Oceania are catching up rapidly. Asia and Africa are currently home to 80% of all twin deliveries in the world. LIMITATIONS, REASONS FOR CAUTION: For some countries, data were derived from reports and papers based on hospital registrations which are less representative for the country as a whole than data based on public administrations and national surveys. WIDER IMPLICATIONS OF THE

findingsThe absolute and relative number of twins for the world as a whole is peaking at an unprecedented level. An important reason for this is the tremendous increase in medically assisted reproduction in recent decades. This is highly relevant, as twin deliveries are associated with higher infant and child mortality rates and increased complications for mother and child during pregnancy and during and after delivery. STUDY FUNDING/COMPETING INTEREST(S): The contribution of CM was partially supported by the European Research Council (ERC) under the European Union's Horizon 2020 Research and Innovation Programme (grant No 681546, FAMSIZEMATTERS), Nuffield College, and the Leverhulme Trust. The contribution of GP was partially supported by the French Agence Nationale de la Recherche (grant No ANR-18-CE36-0007-07). The authors declare no conflict of interest. TRIAL REGISTRATION NUMBER: N/A.