DNA testing has disclosed that a set of Vietnamese twins have two different fathers. There are only 10 such cases of bipaternal twins reported from across the world so far. — Photo dantri.com.vn

HA NOI (VNS) — A set of twins in the northern Hoa Binh Province were found to have different fathers, following DNA tests done by the Ha Noi-based Centre for Genetic Analysis and Technology.

This is the first case of its kind to be detected in Viet Nam. The two-year-old twins of the same sex look different, as one has thick and wavy hair, while the other has thin and straight hair.

The 34-year-old father, under pressure from a family member, agreed to a DNA test, which proved that he is the father of one infant.

In order to rule out possible mistakes by nurses at the hospital in giving them a baby of another couple, the wife also underwent a DNA test. The result shows that she is the mother of both children.

President of Viet Nam Genetics Association Le Dinh Luong said bipaternal twins were very rare.

There are reportedly about 10 sets of twins with different fathers across the world, including one in Poland, one in Spain, two in the United States and two in India.

He said the phenomenon had a scientific explanation.

Two eggs of the same mother get fertilised by two different fathers, within the same ovulation period.

When we see twins, we assume that both eggs were fertilised during a single act of intercourse. But it's quite possible for one egg to be fertilised during one act of intercourse, and the other during another.

It's only logical, then, that when a woman has sex with more than one man while she's fertile, "heteropaternal superfecundation" can occur. That is, each egg can be fertilised by a different father.

However, if the twins don't look alike, it should not be assumed that they have different fathers. Like siblings, twins with the same father can look completely different from each other. They may even appear to be of different races. — VNS