A rare case of twins who are fathered by two different men has been recorded in northern Vietnam after DNA results revealed only one of the two baby twins is biologically related to the tested father.

This rare physiological occurrence is known scientifically as superfecundation, which is the fertilization of two or more ova (egg cells) from the same menstrual cycle by sperm from separate acts of sexual intercourse. Heteropaternal superfecundation occurs when two different males father fraternal twins.

The case was recorded before Tet, or Vietnam’s Lunar New Year festival, which started on February 8, said Nguyen Thi Nga, director of the Center for Genetic Analysis and Technologies in Hanoi.

A man had come to her facility requesting a DNA test to determine whether he was the biological father of his wife’s newborn twins.

Nga said the man’s relatives had pressured him to take the test because one of the babies looked so different from him and the other one.

The results that came out indicated that he was blood-related to only one of the two babies, while his wife proved to be the biological mother of both.

Nga said this was only the second case of heteropaternal twins that she had dealt with in her whole career.

There have also been only 10 other recorded incidents of heteropaternal superfecundation worldwide.

According to Ho Sy Hung, deputy director of the Center for Reproductive Assistance at the National Hospital of Obstetrics and Gynecology, heteropaternal twins are extremely rare and only known through reports, with no documented statistics.

Theoretically, Hung said, it is still possible in real life if a woman engages in sexual intercourse with two different men and her body releases two ova in that same menstrual cycle.

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