important factor associated with (dizygotic) twinning is maternal age. The number of twin pregnancies increases substantially with maternal age, until age 38, and then decreases again [1,3,11,12]. Other factors associated with twinning are parity (the number of pregnancies experienced by the mother before the twin pregnancy) [1,12,13], maternal height [13–15], smoking [16,17], oral contraceptive use [12,18] and race/ethnicity [1,6,19,20]. There is also a substantial hereditary component, which runs through the female line [21,22]. A major ‘new’ factor influencing twinning rates across the globe is the increasing use of assisted reproductive technology (ART), like in vitro fertilization (IVF), intra-uterine insemination (IUI) and ovulation induction (OI), which are associated with a higher number of twin births [11,23–25]. Twinning rates in high income countries are known to vary considerably over time [3,24–29]. In the USA, Australia and many European countries (e.g. Austria, Czech Republic, Denmark, Germany, Italy, Norway, Sweden, Switzerland) they decreased from around 12 or higher in the 1920s to under 10 in the 1970s and then increased again to values of 13–16 around the year 2000. The initial decline in these countries was partly caused by a decrease in the age at which women have their children [25–27]. In the same way, the recent increase might have partly been caused by the rise in age at childbirth since the 1970s. However, both in Western and in East-Asian countries this increase was also to a considerable part the result of the rise of ART [25,26,28–30]. Methods Data are derived from the Demographic and Health Surveys (DHS), large representative household surveys held since the mid 1980s in many developing countries. The DHS programme is sponsored by USAID and executed by MEASUREDHS, in collaboration with national statistical agencies (www.measuredhs. com). In the DHS surveys all usual resident women aged 15–49 obtain an extensive oral interview in which a complete birth history is collected. Information on all pregnancies that resulted in a live birth was recorded, including whether it was a singleton or multiple birth. We use the birth history data collected in 150 DHS surveys held between 1987 and 2010 in 75 low and middle income countries. To limit the time frame of our figures and prevent selection problems, we restricted our sample to births that occurred in the 10 years prior to the interview. Information was obtained on 2,473,209 births experienced by 1,379,694 women. Of these births, 30,895 were twin births. As China was not represented in the DHS, we computed a comparable figure for this country on the basis of data published by Gan et al. [7] on women interviewed in the 1990 Census of China who gave birth in 1989. The number of births and twin births in China were computed on the basis of Table 1 of Gan et al. (p. 634). Among the reported 23,477,961 Chinese births, there were 186,273 twin births. National twinning rates were computed by dividing the number of twin births by the total number of births and multiplying the outcome by 1000. Given the substantial positive association between twinning and mother’s age at birth [1,12], besides these ‘natural’ twinning rates also rates standardized for mother’s age at birth are presented. To compute these standardized rates, the age distribution of the countries was transposed to a reference population with an age distribution equal to the average age distribution of the 76 countries included in our study. Results The average of the national twinning rates in the 76 countries was 13.1 per 1000 (each country weighted equally) or one twin Twinning across the Developing World birth in 76.3 births. This figure is close to the average rate of spontaneous twinning mentioned in the literature of one twin birth in 80 births [5,10]. National twinning rates for the 76 countries are shown in Figure 1 and in Table S1. The figures are total twinning rates, with monozygotic (identical) and dizygotic (fraternal) twins combined. Given the relative stability of monozygotic twinning rates across human populations [1,8–10], the variation observed among the countries is almost completely due to variation in dizygotic twinning. Figure 1 shows that there is huge variation in twinning rates across the different regions of the developing world. The very low rates observed for the pre-ART period in Japan, Hong Kong, Singapore and Taiwan [1,28] turn out to be the dominant pattern in the whole South and South-East Asian region. China, Vietnam, Thailand, Philippines, Indonesia, India, Bangladesh, Nepal and Kyrgyzstan all have twinning rates below 9 per 1000. Twinning rates are somewhat higher in Pakistan, Sri Lanka, Cambodia, Kazakhstan and Uzbekistan, but also in these countries they remain below 10 per 1000 (see also Table S1). In Africa, Nigeria is dethroned as the twinning champion of the world. The high twinning rates that were known to exist in this country turn out to be the dominant pattern in the whole Central African region. A zone with high twinning rates of above 18 per 1000 runs from Guinea in the West along the Atlantic coast to Congo DR and then crosses the continent to Tanzania, Mozambique and the Comoros. South of this zone, in Namibia, Lesotho, South Africa, and Madagascar, twinning rates are clearly lower with values of 11–15. To the north, we see a gradual decrease, with somewhat lower levels in the Sahel countries and intermediate levels of under 15 in Morocco and Tunisia. A steeper decrease is observed in the north-eastern direction, but there Egypt with a twinning rate of 17.7 is an exception. The highest national twinning rate found in our data is observed in Benin. With 27.9 per 1000, the rate in this country is substantially higher than in any other high-twinning country. Little was known about the variation in twinning rates among Latin American countries. Our data reveal that this continent to a large extent resembles Asia, with low twinning rates of under 9 per 1000 in most of the countries. Only the Caribbean island Haiti has a substantially higher twinning rate of 14.1 per 1000. This is probably due to the high percentage of persons from West-African descent living there. Bolivia (6.7) is, after Vietnam (6.2), the country with the lowest twinning rate in our data. There are three middle income Eastern European countries included in our database, Ukraine, Moldova and Albania. Ukraine and Moldova have a low twinning incidence of 8.9 and 8.2 per 1000 births respectively. Twinning incidence in Albania is with 12 per 1000 at an intermediate level. In the Middle East, Turkey has a relatively low twinning rate of 9.9 and Jordan an intermediate rate of 14.4. Table S1 also presents twinning rates that are standardized for mother’s age at birth, the behavioural factor most strongly related to twinning. Given the differences in age structure among different regions of the developing world, standardized figures might give a better picture of the variation among countries. However, the age standardized figures differ not much from the unstandardized ones. In only 5 of the 76 countries, the difference is more than 1 per 1000 and the largest difference is 1.4 per 1000. Hence the twinning rates presented in Figure 1 are not much affected by variation in mother’s age at birth among the countries. We also computed the number of triplets. There were 370 triplets in 2,473,209 births, or one triplet in 6,684 births. This outcome is reasonable well in line with Hellin’s law [31,32] which says that if the number of twins is one in X, the number of triplets PLoS ONE | www.plosone.org 2 September 2011 | Volume 6 | Issue 9 | e25239