The fertilized ovum normally implants in the uterine fundus. Shown here diagrammatically are several problems with placentation:
Placenta previa results from a very low lying placenta or a placenta which cover the os. Obviously, severe hemorrhage can result with cervical dilation and passage of the baby through the birth canal.
Placenta accreta results from a lack of formation of a normal decidual plate. Thus, the chorionic villi extend into myometrium, and the placenta cannot separate normally following delivery. Severe hemorrhage results.
Abruptio placenta results from premature separation of the placenta prior to delivery, with formation of a retroplacental blood clot. The blood supply of oxygen and nutrients to the fetus is compromised to a greater degree with increasing size of the abruption.