After giving birth to two boys fathered by two different men, one woman went back to the drawing board to see what happened to cause such an incident.

According to Globo, the new mother grew suspicious about the paternity of her twin boys as they approached their first birthday. From there, a test was conducted with the children’s suspected father, ultimately leaving the woman shocked when the test results came back positive for only one of the boys. “I remembered that I had had sex with another man and called him to take the test, which was positive,” said the unidentified woman. “I was surprised by the results. I didn’t know this could happen. They are very similar.”

The 19-year-old’s attending physician said that while the outcome in this case is extremely rare, it is quite possible. Dr. Tulio Jorge Franco broke down what actually happened to cause such a mishap. “It is possible to happen when two eggs by the same mother are fertilized by different men,” he explained. “The babies share the mother’s genetic material, but they grow in different placentas.”

Heteropaternal superfecundation is the medical term coined for the incident that took place. Biomedica describes it as an “extremely rare phenomenon that occurs when a second ova released during the same menstrual cycle is additionally fertilized by the sperm cells of a different man in separate sexual intercourse.” According to Dr. Franco, an occurrence like this is “one in a million” and something he never expected to witness in his lifetime. Furthermore, the doctor claims that there are only about 20 other instances in the world like it. A similar case occurred in 2015 when a New Jersey man was ordered to pay child support for one of two boys because he only fathered one of them.