A young woman who had sex with two different men on the same day gave birth to twin boys with different fathers.

The 19-year-old Brazilian woman, who asked not to be identified, wanted to check who the father of the babies was as their first birthday approached. She took a DNA sample from one of the two men for a paternity test, NYPost reported.

While she expected her twins to have one father, the test results indicated that was not the case. The paternity test was positive for only one of the twins.

"I was surprised by the results. I didn't know this could happen. They are very similar," the woman said.

Doctor Túlio Jorge Franco, the woman's attending physician, offered an explanation for the bizarre situation. While there are many examples of an egg dividing into two after being fertilized by a single sperm, resulting in the birth of identical twins, the young Brazilian woman's case is different and extremely rare, but not impossible.

"It is possible to happen when two eggs from the same mother are fertilized by different men," Franco told a local news outlet, Globo.

"The babies share the mother's genetic material, but they grow in different placentas," the physician said.

Franco revealed it was due to a condition called Heteropaternal superfecundation in which a second ova released during the same menstrual cycle is fertilized by the sperm cells of another man in separate sexual intercourse, according to the journal Biomedica.

Dubbing the situation as "once in a million," Franco added that only 20 such cases have been recorded in the world. The physician told Globo that doctors indeed noted a small difference in size between the twins, which may have happened because fertilization in two eggs happened a few hours apart.

The twins, now 16 months old, are being raised by one of the two men. Details of if and how involved the other father is in their care were not revealed.

Woman delivers fifth set of twins Pixabay