A Vietnamese woman gave birth to a set of twins two years back. After a while, something seemed very fishy as there were stark differences in the twins' appearances. One had thick wavy hair while the other had thin and straight hair.

After being pressured by his family, their 34-year-old father underwent DNA testing to solve this mystery. And the results indicated that the man was related to only one of the babies. To rule out a hospital mix-up, DNA testing of the mother was carried out, which revealed she was the mother of both children. That's when the big shock came.

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The twins, who are now two years old, were born the same day and are the same sex. But have different fathers. But turns out that such a thing is possible.

How?

It is possible for twins to have different fathers in a phenomenon called heteropaternal superfecundation. It 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.

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In this case, the woman released multiple eggs within a week of one another. Within that narrow time frame, two eggs appear to have been fertilized by sperm from two different men. The product: fraternal twins with different fathers.

How rare it is?

Le Dinh Luong, president of the Genetic Association of Vietnam, told AFP that this was the first such case he was aware of in Vietnam, and only seven cases had been reported in medical literature as of 2011.

"This is rare not only for Vietnam, but for the world," he said.

However he declined to give details of his client, who is from the northern Hoa Binh province, due to confidentiality.

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.

(With AFP inputs)