Twinning across the Developing World Figure 1. Twins per 1,000 births in 76 low and middle income countries. doi:10.1371/journal.pone.0025239.g001 is one in X2. With one in 80.05 births being a twin birth (all births in the DHS data taken together), this rule predicts the triplet incidence to be one in 6,408. This is only four percent higher than the incidence actually observed. The pattern of variation in triplet rates across the developing world is rather similar to that observed for twins. The triplet rate is 285 per million births in the high twinning countries of Africa, 155 per million births in the other African countries, 68 per million births in South and South East Asia and 83 per million births in Latin America without Caribbean. The number of quadruplets in our data is 12, which is too low to draw any meaningful conclusion. Trends How did twinning rates in the major regions of the developing world vary over the last decades? The last column of Table S1 provides the answer to this question. It presents the average annual change in twinning rate between the first and the last survey for the 44 countries in our database for which data for at least two points in time are available. In Figures S1, S2, S3, and S4 the trends in these countries are displayed graphically. The changes in twinning rates over time are generally small and not into one specific direction. On all three continents, some countries show an increase, some show a decrease and some are stable. The lack of an increase comparable to that experienced in high income countries over the last decades suggests that the influence of fertility treatments is still low in these countries. Even in India, where wealthier couples are increasingly using these techniques [33], the increase in twinning incidence over a period of 14 years was no more than 0.84 per 1000 births (14*0.06). As the data for each point in time are based on a different DHS survey, the rather stable outcomes suggest that the twinning rates presented in this paper are measured in a reliable way. At the same time, it should be kept in mind that these are national figures and that within countries substantial variation in twinning rates among regions and groups may exist. Such sub-national variation has for example been observed between urban and rural areas in Sweden, Finland and China [7,27], between different departments of France [1], between blacks and whites in the USA [1,12] and between ethnic groups in Nigeria and Singapore [34,35]. Hence changes over time in national twinning rates need not always reflect actual changes in twinning frequencies, but may to a certain extent reflect changes in fertility levels of groups or regions within countries. Discussion This paper contributes to the twinning literature in several ways. First, for the first time representative and comparable national figures have become available for 75 low and middle income countries, for which earlier only non-representative figures or no figures at all were available. Second, for Asia our data reveal that twinning rates are not only low in a few countries, but in the whole Eastern, South-Eastern and Southern region. The very low twinning rate in India contradicts the idea resonating in the literature since the early 1970s [1,5] that twinning rates among the predominantly Caucasian Indian population are at an intermedi- ate level, comparable to that in most European countries. Third, for Africa the myth is broken that twinning rates in Nigeria are the highest in the world. High national twinning rates are found throughout Central-Africa, and in several countries twinning incidence is higher than in Nigeria. The highest national twinning rate is found is Benin (27 per 1000). Fourth, twinning rates in Latin America turn out to be at a similar low level as in Asia. Fifth, our study shows that the Demographic and Health Surveys constitute PLoS ONE | www.plosone.org 3 September 2011 | Volume 6 | Issue 9 | e25239