It's a curious, but little-known fact that the rate of twin births in West Africa is about four times higher than in the rest of the world. The centre of this twin zone is Igbo-Ora, a sleepy southwest town in Nigeria.
More twins are born here than anywhere else on earth, but nobody is quite sure why this town should be more twin prone than any other.
On the road going into the town of Igbo-Ora, there's a large stone plinth, which proudly welcomes visitors to 'the land of twins'. Here there are few households who do not have at least one set of twins.
Nigeria is the most highly populated nation in the world with an estimated 110 million people and population experts say that the country, particularly the southwest, has the world's highest twinning rate.
Whilst recent accurate figures do not exist, a study conducted by a British gynaecologist, Patrick Nylander, between 1972 and 1982 recorded an average of 45 to 50 sets of twins per 1000 live births in the southwest.
In the predominantly rural community of Igbo-Ora, multiple births are celebrated and have, over the generations, been regarded as special gifts from God. Twins are a blessing, with many pregnant women wishing for multiple births.
'Whilst twins could be considered the result of providence, other local people attribute the predominance of twins to tradition. Meanwhile, others believe it to be diet.'
Yam Births
Central to the Yoruba people's diet is the cassava, a plant with a tuber root, which can be eaten in addition to the leaves and flowers.
Research into multiple births carried out at Lagos's University Teaching Hospital has suggested that a high level of a chemical found in the Yoruba women and the peelings of the tuber could account for the high level of multiple births.
A consultant gynaecologist at the hospital explains:
'These substances are usually linked to the release of more than one egg, which usually leads to twin pregnancies. That means that there is possibly an environmental factor that encourages the high level of this chemical substance.'
The doctor, who has witnessed many multiple births at the run-down hospital, continues by linking the staple diet of the women to the prevalence of the chemical:
'Numerous works has shown that the tuber food of the Yoruba women, especially the peelings, contains a very large amount of these chemical substances. These women have a disproportionately large amount of these chemicals in their system and this encourages the release of more than one egg. There is enough reason to believe in this theory.'
So, could it be that eating tuber peelings leads to multiple births? The International Institute of Tropical Agricultural is uncertain.
Following research into the reputed high oestrogen content of a yam like vegetable called agida, Robert Asiedu is sceptical of the tuber link. Citing the specific cultivation of yams in regions of Asia for their contraceptive qualities, in an article forScience In Africamagazine, he commented:
'Nobody has provided any scientific explanation or evidence that could prove that yam consumption can cause multiple births.'
A Special Breed
Igbo-Ora is not a wealthy town, many families rely on farming for their income and resources are scarce.
Bringing up large numbers of children can be a struggle, but in many families the arrival of twins continues to be warmly welcomed as one townsman explains:
'Being a father of twins is a joy. It is a sort of honour, because you know they are a special breed from God.'
www.bbc.co.uk/worldservice/people/highlights/010607_twins.shtml