an important new data source for twin research. The DHS program is still actively running, with currently about 20 surveys in the field. Hence, in the coming years many new comparable datasets with twin data for developing countries will become available. For twin researchers, these data are important, because in these countries natural twinning rates can still be observed. In high- income countries, the twinning picture has been altered strongly by the influence of ART. Data for African countries offer many possibilities for twin research, because of the high proportion of twins available in national representative datasets. For researchers studying monozygotic twinning, data from the low-twinning countries in Asia and Latin America may however be more interesting. With about 4 in 1000 births known to be monozygotic across the globe [1,10], and all monozygotic twin pairs being same-sex pairs, in regions with 8 twins per 1000 births about two- third of all same-sex pairs can be expected to be monozygotic. Regarding the quality of the data, we can conclude from the low within-country variation in twin rates that twinning information derived from DHS data is rather reliable. This does however not imply that our twinning figures are unbiased in all respects. It is important to note that these twin rates refer to live births. Stillbirths are excluded. Among stillbirths, the proportion of twins is probably somewhat higher than among live births, as fetal (and neonatal) mortality is higher among twins. It is thus possible that twin births are underreported in our data. If so, actual twinning rates will be higher than the figures presented here. Given that infant mortality rates are highest in sub-Saharan Africa, the data for Africa might be deflated most. Hence, to the extent that excluding stillbirths plays a role, the differences between twinning rates in African countries on the one hand and Asian and Latin American countries on the other might be even larger than observed in this study. From a health policy perspective, our finding of high twinning rates throughout Central Africa point to the existence of a great public health challenge. Twins are much more vulnerable than singletons. They have lower birth weight, are more often premature, suffer more from obstetric complications, and have a much higher risk of fetal and neonatal mortality [36–40]. With References 1. Bulmer MG (1970) The Biology of Twinning in Man. Oxford: Oxford University Press. 2. Segal N (1999) Entwined Lives: Twins and What They Tell Us About Human Behavior. New York: Dutton. 3. Little J, Thompson B (1988) Descriptive epidemiology. 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Solving this problem calls for investments in screening programs for detecting multiple preg- nancies and in facilities for antenatal, delivery, and postnatal care tailored toward the special needs of these children and their mothers [37–39]. Supporting Information Table S1 Natural and standardized twinning rates, number of twin births, total births, survey years, and annual change for 76 low and middle income countries. (PDF) Twinning rates in Asian countries over time. Twinning rates in Latin American countries Figure S3 Twinning rates in North and countries over time. (PDF) West African Figure S4 Twinning rates in Central and South African countries over time. (PDF) Acknowledgments We are grateful to MeasureDHS (www.measuredhs.com) for making the Demographic and Health Survey datasets available for this research and to Ron Wunderink for preparing Figure 1. Author Contributions Analyzed the data: JS CM. Wrote the paper: JS CM. 14. 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