In 1880, German physician Friedrich Küchenmeister reviewed 47 cases of lithopedia from the medical literature and distinguished three subgroups: "Lithokelyphos" ("Stone Sheath"), where calcification occurs on the placental membrane and not the fetus; "Lithotecnon" ("Stone Child") or "true" lithopedion, where the fetus itself is calcified after entering the abdominal cavity, following the rupture of the placental and ovarian membranes; and "Lithokelyphopedion" ("Stone Sheath [and] Child"), where both fetus and sac are calcified.