Infarcts are areas of dead tissue. Placental infarcts are parts of the placenta where the tissue has died for any number of reasons. If enough of the placenta is lost due to infarction, the fetus will die.
The placenta is often called a disc since it is usually round, so I suppose the pathologist was referring to the placenta that way. Otherwise the term "disc" could mean discolored, as tissue color will lighten after it infarcts and blood no longer circulates in the dead area.
Tissue infarction doesn't happen because of hypertension or preeclampsia, but both of those conditions are often diagnosed when there is placental disease ongoing. Why parts of the placenta infarct is a mystery--possibly due to clotting problems in the mother, abnormal blood vessels, fat/lipid disorders, abnormal genes/chromosomes, sometimes infection.
It might be appropriate to investigate you for any underlying abnormalities of your clotting system.