Most cesarean scar ectopic pregnancies fail during the first 2 months of pregnancy, but ongoing cesarean scar ectopic pregnancies are the precursors of placenta accreta spectrum.

The aim of this study was to evaluate placentation inside a cesarean scar defect of the lower uterine segment and its relation to development of placenta accreta spectrum.

We reviewed the ultrasound and histopathologic findings of 94 patients diagnosed with a first-trimester cesarean scar ectopic pregnancy including 92 managed by transcervical suction curettage and 2 managed by hysterectomy after failed conservative surgery and of 10 patients with ultrasound signs suggestive of accreta placentation during the second (n=2) and third trimester of pregnancy (n=8), managed by cesarean hysterectomy. The ultrasound features of both groups were compared with general histopathological examination. Cytokeratin 7 immunostaining was used to evaluate the migration pattern of extravillous trophoblast in 13 cases of live cesarean scar ectopic pregnancies after surgical evacuation and samples from 9 hysterectomy specimens with placental tissue in situ.

In the cesarean scar ectopic pregnancy group there were 61/92 (65%) with a live pregnancy and 33 (35%) with a failed pregnancy and 2 with prolonged retention of placental tissue that required an emergency secondary hysterectomy due to uncontrollable hemorrhage after transcervical suction curettage. All cesarean scar ectopic pregnancies presented with marked dilatation of the vascular network around the gestational sac and 32 (34%) cases were reported as presenting with abnormally increased vascularity. Anchoring villi directly attached to the decidua were found in only 7 (11.5%) live pregnancies and not in failed pregnancy. Histopathological examination in failed cesarean scar ectopic pregnancies showed signs of villous retention and degeneration including fibrotic and edematous villi and diffuse perivillous fibrin deposition following embryonic demise but no signs of aneuploidy or trophoblastic hyperplasia. Extravillous trophoblast cells arranged in deep confluent sheets or scattered deep inside the decidua below the anchoring villi were observed in 4 cases of live cesarean scar ectopic pregnancies and deep inside the scar area in 2 samples from the hysterectomy specimens. In second trimester placenta accreta spectrum, there was increased subplacental vascularity and intraplacental lacunae in both cases. Extended remodeling of the lower uterine segment and anomalies of the utero- and intraplacental circulation were found on preoperative ultrasound examination in all third-trimester placenta accreta spectrum cases. All samples (n=35) from the accreta area in cesarean hysterectomy specimens showed histological evidence of myometrial thinning, disarray of myofibers and tissue edema. In 28 of these samples, thick fibrinoid depositions between the scar and the villous tissue were found. Immunostaining of accreta areas showed extravillous trophoblast cells deep below the placental basal plate. They reached but did not breach the uterine serosa in any case.

Accreta placentation is a progressive phenomenon where villi become abnormally attached inside the myometrial scar, requiring surgical removal. While the adherent villi give rise to large numbers of migratory extravillous trophoblast, the placenta does not spontaneously breach the outer layer of the scar even when it bulges outside the normal uterine boundary.