Vietnam has identified an extremely rare case of bi-paternal twins — twins with different fathers — a professor at a DNA testing lab in Hanoi said.

Key points: Twins are tested because of different appearances

Twins are tested because of different appearances Results show twins are bi-paternal

Results show twins are bi-paternal This happens when two eggs are fertilised by two different men

This happens when two eggs are fertilised by two different men Similar case reported in US

A set of twins born to a Vietnamese couple, whose names have not been released, were recently taken for testing because of stark differences in the children's appearances, according to local reports.

"Our Centre for Genetic Analysis and Technology lab has tested and found a pair of bi-paternal twins," Le Dinh Luong, president of the Genetic Association of Vietnam, said.

"This is rare not only for Vietnam, but for the world," he said, explaining that the two children had different fathers but the same mother.

This can happen if two eggs from the same mother are fertilised by sperm from two different men during separate acts of sexual intercourse within the same ovulation period.

According to the state-run Tuoi Tre newspaper, the Vietnamese husband requested genetic testing at the Hanoi lab to determine whether he was the biological father of both his wife's twins.

The man's relatives pressured him to take the test because one of the babies looked strikingly different from him and the other child, the report said.

The results indicated the man was related to only one of the babies.

Ruling out a hospital mix-up, his wife was found to be the biological mother of both children, Tuoi Tre reported.

According to Dan Tri online news site, the twins were the same sex, born on the same day, just a few hours apart, but looked completely different.

The report said the twins were now two years old and one of them had thick, wavy hair, while the other had thin, straight hair.

Another rare case of bi-paternal twins was reported in the United States last year, when a court ruled that a man was only required to pay child support for one girl in a set of twins after DNA tests proved he was not the father of both.

AFP