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When it comes to having twins, not all regions are created equal. Central Africa snags the record for the highest twin birthrate, while Asia and Latin America have much lower rates of twinning, according to a new international study and global twins database.
The central African country of Benin has the highest national average of twinning, with a whopping 27.9 twins per 1,000 births, the researchers added.
The findings may help answer questions about the causes of twinning, which may range from a mother's age, height and diet to genetic factors that are passed down through the maternal line, as well as mortality differences between boys and girls in certain regions. [Countdown: The Science of Twins]
Twins have long fascinated the world, even making their way into myth and religion, from Castor and Pollux, the brothers from Greek mythologyand basis for the constellation Gemini, to the epic tales of the Hero Twins in the sacred ancient Mayan book known as "Popol Vuh." Identical twins have proven vital in science as well, in particular with questions about nature and nurture — since they are genetically identical, any differences seen between them can reveal the effects that environment might have on individuals.
Until now, scientists had a very incomplete picture of the number of twins around the world. Reliable national information on twinning was only available from highly developed countries with good birth registrations. Data from less developed regions were often weak or lacking all together.
Now scientists have created twins databases for 76 developing countries, the most comprehensive yet for the developing world. This includes data on approximately 2.5 million births by nearly 1.4 million women collected between 1987 and 2010.
Of the developing nations studied, 13.6 twins per 1,000 births were born on average. This is comparable with intermediate rates seen in the United States, Australia and many European countries of nine to 16 per 1,000 births.
The highest twinning rates were seen in Central Africa, with more than 18 twins per 1,000 births. The especially high twinning seen in Benin might be linked to the Yoruba ethnic group, which can be found in Benin as well as in Nigeria and Togo, said researcher Jeroen Smits, a sociologist and economist at Radboud University Nijmegen in the Netherlands.
In comparison, twinning is very low in Asia and Latin America, with a rate often less than eight per 1,000 births. The major exception are the Caribbean Islands, where many people of African descent live — for instance, Haiti had 14 twins per 1,000 births.
Reasons behind twin rates
It remains a mystery as to why there are vast differences in twin birth rates among the different regions of the developing world. Past research suggests that identical twins occur at a relatively constant rate of 3.5 to 4 per 1,000 births globally. Instead, most variations in twinning worldwide seem due to birth rates of fraternal twins.
A key factor linked with fraternal twinning is a mother's age — the number of twin pregnancies rises substantially with maternal age and then decreases after age 38. The number of pregnancies a woman had before a twin pregnancy also appears to play a role, as might details such as smoking, contraceptive use and even a woman's height — the taller you are, the greater your chances seem to be of giving birth to fraternal twins. In addition, there is a substantial hereditary component to fraternal twinning that apparently runs through the female line.
The new twins database offers many opportunities for further research into why twinning varies worldwide, especially by focusing on differences among regions and ethnic groups in central Africa, Smits said.
"If associations with diet habits exist as has sometimes been suggested, or other characteristics of the living circumstances play a role, they may be revealed by analyzing the variation between groups and regions that is observed," Smits told LiveScience.
Prior studies have shown twinning rates can vary considerably over time in developed nations. For instance, in the United States, Australia and many European countries, they decreased from about 12 or higher per 1,000 births in the 1920s to fewer than 10 per 1,000 births in the 1970s and then increased again to values of 13 to 16 in about 2000. The initial decline and recent increase may partly have been due to a decrease and increase in the age at which women had their children, respectively, although this modern rise might also be due to recent fertility technologies. However, this new research on the developing world did not show any large changes either up or down.
The researchers cautioned their data was based on live births. Since twins are susceptible to higher mortality rates during pregnancy and birth, the actual number of twins could be somewhat higher. "With child mortality levels being highest in Africa, this would imply that actual twinning rates in Central Africa would be even higher than those observed," Smits said.
This data is part of research on inequality between girls and boys in developing countries in terms of education, infant mortality, health and related factors, for which the scientists have gathered demographic, socioeconomic and health data on millions of households in more than 100 countries.
"Girls are still very much disadvantaged in some parts of the world," Smits said. "This database offers many opportunities to study where and why disadvantage is greater and where it is getting smaller."
"The twin database, for instance, allows us to look at mortality difference between boys and girls," he added. "If they are born at the same time in a family, the parents directly have to choose between investing resources in one or the other — hence if twin girls suffer more from child mortality or have less chances to go to school than their twin brothers, this is a strong sign of gender discrimination."
Smits and his colleague sociologist Christiaan Monden detailed their findings online Sept. 28 in the journal PLoS ONE.
