Twinning across the Developing World Jeroen Smits1*, Christiaan Monden2 1 Nijmegen Center for Economics, Institute for Management Research, Radboud University Nijmegen, Nijmegen, The Netherlands, 2 Department of Sociology, University of Oxford, Oxford, United Kingdom Abstract Background: Until now, little was known about the variation in incidence of twin births across developing countries, because national representative data was lacking. This study provides the first comprehensive overview of national twinning rates across the developing world on the basis of reliable survey data. Methods: Data on incidence of twinning was extracted from birth histories of women aged 15–49 interviewed in 150 Demographic and Health Surveys, held between 1987 and 2010 in 75 low and middle income countries. During the interview, information on all live births experienced by the women was recorded, including whether it was a singleton or multiple birth. Information was available for 2.47 million births experienced by 1.38 million women in a period of ten years before the interview. Twinning incidence was measured as the number of twin births per thousand births. Data for China were computed on the basis of published figures from the 1990 census. Both natural and age-standardized twinning rates are presented. Results/Conclusions: The very low natural twinning rates of 6–9 per thousand births previously observed in some East Asian countries turn out to be the dominant pattern in the whole South and South-East Asian region. Very high twinning rates of above 18 per thousand are not restricted to Nigeria (until now seen as the world’s twinning champion) but found in most Central-African countries. Twinning rates in Latin America turn out to be as low as those in Asia. Changes over time are small and not in a specific direction. Significance: We provide the most complete and comparable overview of twinning rates across the developing world currently possible. Citation: Smits J, Monden C (2011) Twinning across the Developing World. PLoS ONE 6(9): e25239. doi:10.1371/journal.pone.0025239 Editor: Marie-Louise Newell, Africa Centre for Health and Population Studies - University of KwaZulu-Natal, South Africa Received June 28, 2011; Accepted August 30, 2011; Published September 28, 2011 Copyright: 2011 Smits, Monden. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: The authors have no funding to report. Competing Interests: The authors have declared that no competing interests exist. * E-mail: j.smits@fm.ru.nl Introduction Scientists and non-scientists alike are fascinated by twins. They speak to us about the uniqueness of a human individual and the natural bond between siblings. In some societies twins are revered and in others they are looked upon with suspicion. At the same time, twin studies are fundamental to the scientific understanding of the role of nature and nurture [1,2]. Yet surprisingly, up to now, we have had a very incomplete picture of the number of twins around the world. Only for highly developed countries with good birth registrations, reliable national information on the incidence of twinning and the changes therein over time is available. Information for less developed regions is weak or lacking all together. Since the early 1970s, several overview studies have been published in which figures from a large number of smaller studies were brought together [1,3–6]. The overall conclusion drawn from these figures was that natural twinning rates were low in East Asia and Oceania (less than 8 twin births per 1000 births), intermediate in Europe, USA and India (9–16 per 1000 births) and high in some African countries (17 and more per 1000 births). However, for the less developed parts of our world, this conclusion is based on low quality and unrepresentative data for only a handful of countries. Most data were derived from local birth registers, which often are of dubious quality, or from hospital registrations, which are notorious for their selectivity problems. In hospital data, twin births may be either overrepresented, because twin pregnancies are more often associated with complications, or underrepresent- ed, when complicated cases are referred to special hospitals or only first pregnancies are accepted [3,5]. Comparable and representa- tive national information on the incidence of twin births in developing countries has been lacking up to now. The current study fills this gap in our knowledge by providing the most comprehensive overview of twinning in the developing world currently possible. This overview is based on a unique data source that has not yet been used for comparative twin research: The large representative household surveys collected since the mid-1980s as part of the Demographic and Health Survey (DHS) programme. Using these data, we compute national twinning rates for 75 low and middle income countries. A comparable twinning rate for China is additionally computed on the basis of published data [7]. For 44 countries, data for more points in time is available, so that changes over time can be studied. Sources of variation Differences in twinning rates among countries and over time are mostly due to variation in dizygotic twinning. Monozygotic twinning is thought to occur at a relatively constant rate of 3.5– 4 per 1000 births across human populations [1,8–10]. The most PLoS ONE | www.plosone.org 1 September 2011 | Volume 6 | Issue 9 | e25239