'Extremely rare' twins born with different dads
A couple whose twins look so different it was feared one may have been accidentally swapped at birth have found that the babies actually have different fathers.
The twins are the same sex, born on the same day, just a few hours apart, but look completely different, Dan Tri online news site reports.
Their father was pressurised into testing the children by family members, who believed their appearances - one has thick curly hair, the other has fine straight hair - meant there had been a mix-up at the hospital.
But DNA testing has found that the babies, born in Vietnam two years ago, are bi-paternal twins - sharing the same mother, but different fathers, CNN reported.
Scientists at the Center for Genetic Analysis and Technologies in Hanoi believe it is the country's first case of the "extremely rare" phenomenon.
Le Dinh Luong, president of the Genetic Association of Vietnam, said the family have been surprised by the news, but are looking at the best way to deal with the situation.
A confidentiality agreement with the couple prevents any further details being released.
There are thought to be about 10 sets of bi-paternal twins across the world, including one in Poland, one in Spain, two in India and two in the US
Last year a New Jersey man was ordered to pay child support for one girl in a set of twins, after genetic tests showed he was not the father of the other.
How does it happen?
A woman's egg has a life span of 12 to 48 hours and a sperm can live for seven to 10 days.
This leaves a window of around a week time for potential overlap and the fertilisation of two eggs by two sperm from separate instances of sex.