Lithopedions occur from abdominal ectopic pregnancies. There are two types of abdominal pregnancies primary and secondary. In the primary abdominal pregnancy, the fertilized egg is transported backwards into the peritoneal cavity where it implants onto an organ. The secondary abdominal pregnancy occurs when there is a tubal ectopic pregnancy and the uterine tubes rupture. This allows the placenta to spread into the abdomen, causing an abdominal pregnancy.
The primary abdominal pregnancy is the rarer of the two, but is how lithopedions are formed. In the primary abdominal pregnancy the placenta is extended which can cause extreme bleeding. Many times the fetus dies because of lack of blood supply from the extended placenta, or because there is no exit route from the abdominal cavity to the external world. If there is not severe hemorrhaging causing death to the mother, but the fetus dies, the mother’s body can calcify the dead fetus if it is not removed. This causes a lithopedion baby (Gray, 1975).
The picture below shows two different ultrasounds. The first, labeled A, is of a primary abdominal ectopic pregnancy. The second, labeled B, is a secondary ectopic pregnancy. From this you can see the difference of the two types of abdominal pregnancies, based on the ruptured uterine tube.