This section is from the book "A Manual Of Pathology", by Guthrie McConnell. Also available from Amazon: A Manual Of Pathology.
If there is any interference with the entrance of the impregnated ovum into the uterus an extrauterine development takes place. This may be within the ovary, between the tube and ovary - tubo-ovarian - or, what is most common, within the tube - a tubal or ectopic pregnancy.
The chorionic villi are formed, decidual develop, and a placenta is evolved. At the same time there is commonly a decidua formation within the uterine cavity. As the embryo increases in size the walls of the tube become gradually thinner. By about the third month the tube generally ruptures. This may take place within the layers of the broad ligament, into the peritoneal cavity, or into the uterus. In any case there are severe symptoms of pain and shock and large internal hemorrhage occurs. It is very dangerous if the rupture has taken place at the placental site. Death may result from the loss of blood or from a peritonitis.
If the ovum has died the fetus may degenerate and become infiltrated with lime salts, forming a lithopedion.
If the impregnated ovum lodges somewhere on the peritoneum, we have an abdominal pregnancy. A similar condition is present at times when the ovum has escaped from a ruptured tube. In such instances the placenta usually remains within the tube. Peritonitis commonly ensues, the fetus perishes, and a lithopedion may form.