Nine months after having sex with two different men on the same day, the 19-year-old mother gave birth to the “one-in-a-million” set of twins.

As her twins’ first birthday drew closer, a Brazilian mother began to have doubts about the children’s biological father. Nine months before they were born, she’d had sex with two different men on the same day, and she wasn’t entirely sure which of the men had fathered the infants.

To confirm her suspicions, the New York Post reported, she took a paternity test, but the results weren’t quite what she’d expected. They came back as a match — for just one of the twins.

“I remembered that I had had sex with another man and called him to take the test, which was positive,” the new mother, who asked to remain anonymous, told local news outlets. “I was surprised by the results. I didn’t know this could happen, and the babies are very similar.”

Evidently, this sort of thing can happen, though it is incredibly rare.

“It is possible to happen when two eggs from the same mother are fertilized by different men,” Dr. Tulio Jorge Franco, the woman’s attending physician, said. “The babies share the mother’s genetic material, but they grow in different placentas.”

The technical term is “heteropaternal superfecundation,” but to put it simply, the extremely rare phenomenon occurs when two eggs are released during…