Nope. You’re not seeing double. You’re just seeing twins. And they seem to be virtually everywhere in West Africa, especially in Igbo-Obra, a quiet farming community in Nigeria. A sign proclaiming “The Nation’s Home of Twins” welcomes visitors at the entrance of the town. According to the community leader, “There is hardly a family here without a set of twins.” He, himself, had three sets but only one pair survived, whereas his father had 10 sets of twins.
Double pregnancies occur either when a fertilized egg is implanted in the uterine wall and develops into a zygote which splits into two embryos, in which case we call the offspring “identical twins;” or when two eggs are produced and fertilized and implanted in the uterine wall separately, in which case we call the twins “fraternal” or “non-identical twins.” Identical twins are a random event that happens in about 0.3% of pregnancies throughout the globe. Fraternal twins, on the other hand, are a different issue. Incidence rates vary from region to region and average to about 2.3% in the world. That number is higher in the West and will probably keep rising due to the advent and increasing use of fertility drugs and in vitro fertilization, both of which raise the chance of double or even multiple ovulations.
But, even without fertility drugs or in vitro fertilization, West Africa takes the crown for the highest twin birth rate in the world of nearly 5%, particularly with a much higher incidence of fraternal twins than the global average. This is especially true for the Yoruba people in Nigeria who mostly live in Igbo-Obra, the southwestern part of the country. This land of twins has fertility experts completely baffled, and local residents are equally nonplussed. The most popular theory, although not accepted by everyone, is that it is due to the people’s diet.
Central to the Yoruba people’s diet are plants with a tuber root, such as yam and cassava. Some residents and experts in Nigeria believe that, due to the high phytoestrogen content in these plants, women are more likely to release more than one egg at a time, which usually leads to twin pregnancies. Phytoestrogens are chemicals similar to estrogen from a plant origin. They are found in many herbal products that are advertised to relieve menopause symptoms, enhance breast size, and alter other hormone-related effects. However, there is not really any scientific evidence that these plants are therapeutic in humans or that yam consumption can cause multiple births. In fact, many experts are skeptical about this theory, including Robert Asiedu of the International Institute of Tropical Agriculture, following research into the reputed high estrogen content of a yam-like vegetable called agida.
It is a misconception that yam contains any actual hormone, although it has been suggested that some wild yam creams or supplements might be tainted with artificial progesterone. What the tuber does contain is diosgenin which can be used to commercially synthesize progesterone, cortisone, and other steroids. However, diosgenin cannot be converted into hormones in the body. The common belief that wild yam contains hormones probably comes from the historical fact that progesterone, androgens, and cortisone were chemically manufactured from Mexican wild yam in the 1960s.
So if it is not the yam, why are Yoruba women giving birth to so many twins? Genetics is a possible explanation, as is believed by the chief nursing officer at the hospital Muyibi Yomi, who records a monthly average of five twins for every 100 births and notices that the trend runs in families.
Whatever the explanation, this should be good news for the Yoruba people, for whom twins are regarded as a special gift from God and bearers of good luck. Interestingly, twins are also believed to share one soul between them. If one twin dies in a Yoruba family, a wooden figure called an “ibeji” is to be carved and replace the dead twin. The ibeji is thought to harbour the half soul of the dead twin and is clothed, “fed” and carried by the mother exactly in the same way as the living twin. When the living twin reaches maturity, he becomes responsible for the care of his other half-soul.