Scientists in the Netherlands have solved the medical mystery of what causes some twins to be born identical. It is because these identical twins form after a fertilised egg called a zygote, which splits into two embryos sharing the same genes. 

However, the reason for the split is unknown.

Published in the journal Nature Communications, the research looked at the epigenetic modifications in twins’ DNA, factors that can switch genes “on” or “off” without changing their underlying sequence. 

It was concluded that identical twins from across the world shared similar marks at 834 points across their genome, which happens to be the total of an organism’s DNA.

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As per the scientists, the shared marks can allow researchers to determine, with up to 80 per cent accuracy whether an individual is an identical twin. 

Nancy Segal, a developmental psychologist at California State University, told the Science News publication that it was “a very, very important finding”, as identical twins are predisposed for a variety of conditions, including spina bifida.

However, it is yet to be proved that the chemical marks on the DNA are the cause of identical twins being conceived. 

Also, as many as 12 per cent of human pregnancies may start as multiple embryos but just under 2 per cent carry to term, with many people unaware of the fact that they had a twin.