Twins with different fathers is possible, but very rare. The phenomenon is called heteropaternal superfecundation. There are less than 10 known cases of twins with different fathers in the world. Heteropaternal superfecundation can occur if a woman releases two eggs in one ovulation period and they are fertilized by sperm from different men. It can also happen if she ovulates twice within a short period of time.
Recent case in the Vietnam
A Vietnamese family discovered in 2016 that their fraternal twins are twins with different fathers. This was reported by BBC News in March 2016. The twins had gone through a DNA-test, as relatives pointed out that the 2-year old twins looked remarkably different, and that one of them looked like neither of the parents. A DNA-test ruled out a possible hospital mix-up as both the children were offspring of their mother.
Twins and half-siblings
Twins with different fathers share approximately 25 percent of DNA. Genetically, they are half-siblings. Experts believe that there are more cases of heteropaternal superfecundation than what’s known. Some families are not aware of the fact that there are twins with different fathers in their family and some choose not to act upon their suspicion. The phenomenon has also become more common with the spread of assistive reproductive technologies, said Jennifer Wu, an obstetrician-gynecologist at Lenox Hill Hospital in Manhattan, to the New York Times. This is because men in gay couples sometimes both contribute sperm to a pregnancy.
Recent case in the U.S.
One case of heteropaternal superfecundation in the U.S. was detected through paternity testing for child support. This occurred in 2015 in New Jersey. An alleged father was being sued for child support. He had to go through a paternity test. The test established that he was the father of one twin and not the other. The court ruled that he only had to pay child support for the child he had fathered.