Studies Show that Nigeria Has The Highest Twinning Rate In The World

   A multiple pregnancy means that a woman has two or more babies in her uterus. These babies can come from the same egg or from different eggs. The occurrence and frequency of twinning, however, varies across human populations. The maternal age, socio-environmental factors, increase in the use of contraceptives, the race of human population, increase in the spontaneous abortion rate, and seasonal variations are among the factors that could influence twinning rate.

 
Information on twinning rates in southwest Nigeria is limited.

Delivery Rate   :
     A frequency of twin births of 46.5 per 1000 deliveries and 46.2 per 1000 deliveries was recorded for Ilesa and Ile-Ife respectively.
    The incidence of twinning among the races of the world has been extensively studied. Worldwide changes have occurred in the pattern of twinning rates in recent decades. The highest twinning rate is observed to occur among the Negroids, while the lowest occurred among the Mongoloid population. In Nigeria, the incidence of twinning was estimated to be 45‰.
Bulmer recorded 44.9‰ incidence of twinning in Ibadan, while 48.3‰ and 57.2‰ incidences of twin births were respectively obtained for Igbo-Ora and Ibadan between 1967 and 1969. On an average, the highest record of twinning is among the Yorubas of southwest Nigeria, with approximately 50-53 twin births per 1000 deliveries.These rates were suggested then as the highest in the world. Between 1985 and 1989, 68.1‰ twin births were recorded for Igbo-Ora, while 37.2‰ and 26.9‰ twin births were recorded for Ibadan and Lagos respectively.


Studies  determine  much of a particular trait is attributable to genetic predisposition. Apart from genetic predisposition, a factor that may have also been influencing high twinning rate in southwest Nigeria is diet. There is a general belief that the Yoruba's predisposition to high twinning rate is due to consumption of yam (Discorea sp.), which is believed to contain a natural hormone phytoestrogen, which may stimulate multiple ovulation. Indigene of the studied areas are known to have preference for food prepared in different forms from yam. Maternal history of twinning (though not considered in this study) might also have contributed to the high incidence of twinning observed in this study and previous studies from southwest Nigeria. This is because the probability of a subsequent twin pregnancy is increased fourfold in mothers of twins, and the risk of having dizygotic (DZ) twins is roughly double for a woman whose mother or sister has DZ twins.

Links:
http://www.webmd.boots.com/pregnancy/guide/twin-pregnancy

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3150208/
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2840794/
http://www.drugs.com/mtm/wild-yam.html
http://www.bbc.co.uk/worldservice/people/highlights/010607_twins.shtml
http://www.randafricanart.com/Yoruba_Customs_and_Beliefs_Pertaining_to_Twins.html