Placenta accreta

(A) Villi have implanted in scar tissue (arrows), which replaced the endometrium in an area of thinned myometrium. Placenta accretas often occur at the site of prior cesarian sections.
(B) Note the layer of dense pink fibrinoid covering the scar (dashed arrow). The presence of fibrinoid material between villi and myometrial tissue does not exclude a diagnosis of placenta accreta.
(C) Villi are immediately adjacent to underlying myometrium without any intervening decidua.
(D) Villi in direct contact with myometrium (thick arrow). Note the implantation site trophoblast (arrowhead), which can be mistaken for decidua.