Around five years ago, a pregnant woman in Australia went in for her six-week ultrasound and was told she would be having twins. Her scan showed that the fetuses were sharing a single placenta, indicating that they were identical. But when the woman came in for a follow-up ultrasound at 14 weeks, it was discovered that she was carrying a boy and a girl—something that is impossible in identical twins.

In a paper published in the Journal of New England Medicine, doctors reveal that the twins were sesquizygotic, or “semi-identical”—a very rare phenomenon that has been documented only one other time, according to Reuters.

Typically, twins are conceived in one of two ways. Either they are identical, meaning that a single egg fertilized by a single sperm splits and develops into two fetuses, or they are fraternal, meaning that two eggs are each fertilized by a single sperm and develop simultaneously in the womb. But doctors think something else happened in the conception of the Australian twins: the mother’s egg was fertilized by two different sperm.

The study's lead author Michael Gabbett, a clinical geneticist at the Queensland University of Technology, explains that when this type of conception happens, it produces three sets of chromosomes—one from the mother and two from the father—instead of the usual one set each from mom and dad. This would normally result in a miscarriage, but in the case of the Australian twins, the fertilized egg formed three cells: one had DNA from the egg and the first sperm, the second had DNA from the mother and the second sperm, and the third had DNA from the two sperm. Because humans need chromosomes from both parents to survive, the third cell eventually died. But the remaining cells “would go on to combine together and then divide again into the two twins,” writes Yasemin Saplakoglu in Live Science.

This in turn means that the Australian babies have a different genetic makeup than what is typically seen in twins. Identical twins share the same DNA (though changes in chemical markers can affect how those genes are expressed); fraternal twins share around 50 percent of their DNA, the same as any other siblings. The semi-identical twins, on the other hand, were found to be “100 percent identical on the mother’s side and 78 percent identical the father’s side, so this averages out to being 89 percent identical,” Gabbett tells Gizmodo’s Ed Cara.

Wondering if there were other cases of semi-identical twins that had gone unreported, Gabbett and his team combed through previous medical studies and an international genetic database of 968 fraternal twins and their parents. They found just one other instance of semi-identical twins, which was reported in the United States in 2007. In that case, doctors didn’t realize that the twins were sesquizygotic until after they were born, when one of them was found to be intersex, meaning that they had ambiguous genitalia.

The Australian twins are anatomically male and female, but according to Cara, they both have male and female sex chromosomes. A baby’s sex is determined by sperm that carry either an X or Y chromosome; generally speaking, females have two X chromosomes (XX) and males have an X and a Y chromosome (XY). One of the Australian twins, however, has around a 50-50 split of XX and XY chromosomes, while the other has a 90-10 split of XX to XY. The girl has had her ovaries removed because doctors observed “some changes in her ovary that people weren’t comfortable with,” Gabbett tells Reuters. “The boy is continuing to have his testes monitored.”

Shortly after birth, the girl also developed a clot that cut off blood supply to her arm, which resulted in the limb being amputated. That complication, however, is not believed to be connected to the her unusual conception. And aside from those setbacks, the twins, who are now four and a half years old, appear to be healthy and doing well.