Introduction Placenta accreta is an increasingly prevalent cause of maternal morbidity/mortality. Decidua is, at least focally, defective and abnormal uterine invasion by extravillous trophoblast (EVT) has been proposed.

Aim Examine EVT invasion and spiral artery (SA) remodelling in placenta accreta.

Methods 16 (12 accreta, 4 non-accreta) caesarean hysterectomy specimens were studied using immunohistochemistry. Invasive EVT and multinucleate trophoblast giant cells (MTGC) were quantified and confluent EVT (>5 opposed EVTs) assessed semi-quantitatively in 5×100 microscope fields/block (312 fields). In six cases, accreta was focal (i.e., normal decidua in some areas), so sections were assessed for decidua. Arterial transformation was assessed in 274 SA.

Results EVT cell numbers were increased in accreta (57, IQR 29–90) versus non-accreta cases (19, IQR 14–32; p<0.0001); other variables did not differ between non-accreta cases and areas in focal accreta cases showing normal decidua. The % of MTGC was reduced in areas without decidua compared to those with decidua (0.48 vs 23.6; p<0.0001) and EVT was more confluent (62.7% vs 7.5%; p<0.0001). Complete SA transformation was reduced without decidua (32.1% vs 65.4%; p<0.0001).

Discussion Overall numbers of EVT are increased in placenta accreta. Trophoblast giant cells and SA transformation are reduced in accreta areas with no decidua. Our results emphasise the crucial role of decidua in control of trophoblast invasion and SA transformation. The presence of decidua in focal accretas may help explain continued placental function leading to good fetal outcome in these cases.