Prof. Le Dinh Luong, president of the Genetic Association of Vietnam, says DNA testing at his Hanoi lab confirmed the twins have different fathers, the first such case that he knows of in Vietnam, with only seven such cases reported in the world as of 2011.

Luong declined to give details because of confidentiality with his client.

Online newspaper Dan Tri reported that a 34-year-old man from northern Hoa Binh province had DNA testing after being pressured by his family because the twins did not look alike.

One has thick wavy hair while the other has thin and straight hair.

To rule out a hospital mix-up, DNA testing of the mother showed that she was the mother of both children, the report said.

The twins who are now two years old were born the same day and have the same sex, it said.

Bi-paternal twins could happen when the mother has sex with two different men within one to seven days in an ovulation period, says the president of the Royal Australian and New Zealand College of Obstetricians and Gynaecologists, Professor Michael Permezel.

“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,” Prof Permezel told news.com.au.

While bi-paternal twins are very rare, Prof Permezel says he has seen some cases in Australia.

“In the couple of cases I’ve seen, the woman wasn’t married to either man,” he said.

“In an established marital relationship it’s extremely uncommon that the husband isn’t the father.”