A couple have found out their twins have different fathers.

A couple who feared that one of their twins may have been swapped at birth because the babies looked so different have been surprised to hear that their twins have different fathers.

The twins are the same sex, born on the same day, just a few hours apart, but look completely different, CNN reports.

The father was pressured into testing the children by family members, who believed their appearances – one has thick curly hair, the other has fine straight hair – was cause for concern. He underwent DNA testing to try and prove the hospital had given them the wrong baby – but the results proved the opposite.

The baby was indeed one of his twins, but he wasn’t the father.

The babies, born in Vietnam two years ago, are bi-paternal twins – sharing the same mother, but different fathers.

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.

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 US 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.

Dr. Keith Eddleman, director of obstetrics at Mount Sinai Hospital in New York told CNN that a woman’s egg has a life span of 12 to 48 hours, and a sperm is viable for seven to 10 days, so there’s about a week’s time for potential overlap and the fertilization of two eggs by two sperm from separate sexual encounters.