A 19-year-old woman from Brazil has given birth to twin babies who have different biological fathers.

This extremely rare phenomenon occurs when two eggs are released during ovulation. Each ovum would then be fertilised by sperm cells during two separate sexual encounters. In this woman’s case, the second egg was fertilised by another partner that she had sex with at that time.

The woman, who wishes to remain anonymous, gave birth to her twins nine months ago. As the children grew and their features and mannerisms developed, she began to have doubts as to who their father might be.

Opting to take a paternity test, the results came back as a match for only one of her twins.

“I remembered that I had had sex with another man and called him to take the test, which was positive,” the new mum told local news outlets.

“I was surprised by the results. I didn’t know this could happen, and the babies are very similar.”

“It is possible to happen when two eggs from the same mother are fertilised 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 (sic) different placentas.”

Franco described the phenomenon as “one in a million” and said he believed there are only about 20 other instances of the same thing happening in the world. Other doctors, however, estimate that there are around 400 existing cases. While his expression of “one in a million” is a common thing to say, this sort of birth is actually closer to one in a billion.

These children are technically fraternal twins, as two eggs were fertilised from two sperm. Fraternal twins don’t necessarily need to be conceived by one person’s sperm, although that is usually the case.

They are also half-siblings, as well as twins.

Simply incredible!