They were born on the same day, to the same mother, but when it came to identifying their father, an extremely rare discovery has been made in a set of Vietnamese twins.

The twins are bi-paternal, meaning they have different fathers. The rare occurrence is said to be the first case in Vietnam, and there have been only seven cases of bi-paternal twins reported worldwide since 2011.

Prof. Le Dinh Luong, president of the Genetic Association of Vietnam, says DNA testing at his Hanoi lab confirmed the twins have different fathers, although few details have been released to media in order to protect the family's privacy.

A Vietnamese online newspaper has reported that the 34-year-old father had DNA testing after his family started questioning why his twins did not look anything alike.

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The two-year-old twins are the same sex, and one has thick wavy hair, while the other has fine, straight hair.

To rule out a hospital mix up, the babies' mother underwent a DNA test, which confirmed she was the mother of both children.

President of the Royal Australian and New Zealand College of Obstetricians and Gynaecologists, Professor Michael Permezel, told news.com.au while bi-paternal twins are rare, he has seen some cases in Australia.

Bi-paternal twins could happen when the mother has sex with two different men within one to seven days in an ovulation period, Permezel said.

"She might ovulate two eggs that month. One is fertilised by one man and the other is fertilised by another man. Sperm can survive for up to five days, so they could be fertilised on different days." he said.