A Polish archaeological research team says it has uncovered the first case of a “pickled” fetus inside an ancient Egyptian woman.

A research paper published in January by the scientists of the Warsaw Mummy Project revealed that the body belongs to a pregnant woman who died between 20 and 30 years old. The mummy arrived in Warsaw in 1826 inside a sarcophagus. Back then, the mummy was thought to be male, as the coffin’s inscription identified it as a male priest named Hor-Djehuty, adding more mystery to the story. The female mummy may have been stolen or transferred to the priest’s sarcophagus.

The researchers believe that the woman belonged to the high-status community in the Thebes, given the good quality of the body mummification. The woman is also believed to have been between 26 and 30 weeks pregnant, and the fetus had “pickled” inside the womb.

Marzena Ożarek-Szilke, one of the researchers of the Polish team, told the Science in Poland magazine, “During the mummification process, the deceased was covered with natron, a naturally occurring sodium mixture, in order to dry the body. The fetus, however, remained in the uterus and began to ‘pickle’ in an acidic environment. The process was similar to the one thanks to which prehistoric bodies have been preserved to our time in swamps.”