A woman became pregnant with her own child while acting as a surrogate for someone else, touching off a messy situation and drawing attention to the risks of surrogacy. The woman, Jessica Allen, was paid $30,000 to become a surrogate mother for a Chinese couple in April 2016, and when she gave birth to two kids she assumed both of them were theres.

The Chinese couple was thrilled to find out early on in her pregnancy, and even paid her an additional $5,000. But when they were born, it was clear that one baby was Asian, and the other baby was hers. It’s a rare medical situation called superfetation, and it’s only been documented 10 times around the world.

The Chinese couple knew that one of the babies wasn’t theirs, and even wanted to put him up for adoption. Eventually, however, little Malachi was reunited with her mother, who already has two sons with her husband.

“Superfetation is the simultaneous occurrence of more than one stage of developing offspring in the same animal. In mammals, it manifests as the formation of an embryo from a different estrous cycle while another embryo or fetus is already present in the uterus,” reads a Wikipedia description of superfetation. “When two separate instances of fertilisation occur during the same menstrual cycle, it is known as superfecundation.”