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The following text is an excerpt from the book by Elizabeth Bryan Twins,
Triplets and More (the chapter is The Biology of Twinning).
Why do twins happen? The popular explanations for twinning given in different parts of the
world owe much more to imagination than to science. In South Korea I was told that if I ate a
double banana I would have twins. In Malaysia I heard that a double chestnut or millet seed
would have the same effect. Some South American Indians are said to believe that if a mother
lies on her back during labour the baby will split in two.
Nearer home, a tumbler full of water from the spring at St Mungo apparently resulted in
twins for the medieval Scottish. Shock has also been blamed for multiple births. A
contemporary account of an earthquake in Hereford in 1661 reported that shock had caused a
clerk’s wife, Mary Pelmore, to have triplets (all boys and boys, moreover, who had teeth and
spoke as soon as they were born!).
We may realize that ideas of this kind are absurd but so far as identical twins are
concerned, we actually have no more idea of the causes than the most remote Korean villager. It
appears that all women who can conceive have the same chance of having identical twins. The
only exceptions to this are very rare couples where one partner appears to have a dominant gene
for identical twinning running through their family. Anyone would know if they belonged to
one of these rare families as family gatherings would inevitably lead to rather more than the
usual confusion.
Even with fraternal twinning we can never say to a woman, ‘If you become pregnant
you will have twins’, but we can at least say who is most at risk. Having looked at all the known
predisposing factors it seems that if you are longing to have twins it is best to be a 6-foot
Nigerian in your late thirties who already has five children including a set of fraternal twins. If
the last thing you want is twins, it is best to be Japanese, 4 foot 11 inches, aged 20, having your
first pregnancy, with no history of twins in the family.
Race strongly affects the chances of having twins. Black races, and particularly, it
seems, Nigerians, have the highest rates of twinning. The Oriental (or ‘Mongolian’) races have
the lowest rates. Caucasians and Indians are somewhere in between. The incidence of twins is
about five times higher in the Yoruba people of Nigeria than amongst the English and ten times
higher than amongst the Japanese.
Studies have shown that the level of the hormone gonadotrophin is higher in Nigerian
women than in Japanese and those Nigerian mothers who have twins have even higher levels.
Gonadotrophin is a hormone produced by the pituitary gland which stimulates the ovaries to
mature and release their eggs into the fallopian tube. If high levels of gonadotrophins are present
in the blood a second (or more) egg is more likely to be released.
[. . .]
The chance of a mother of any race delivering twins increases until her late thirties.
Parity also has an effect: the more children she already has, the higher her chances of twins.