The Placenta Accreta Spectrum: Pathophysiology and Evidence-based Anatomy for Prenatal Ultrasound Imaging

Abstract The placenta accreta spectrum (PAS) is a complex obstetric complication associated with a high maternal morbidity. It is a relatively new disorder of placentation, and is the consequence of damage to the endometrium-myometrial interface of the uterine wall. When first described 80 years ago, it mainly occurred after manual removal of the placenta, uterine curettage or endometritis. Superficial damage leads primarily to an abnormally adherent placenta, and is diagnosed as the complete or partial absence of the decidua on histology. Today, the main cause of PAS is uterine surgery and, in particular, the uterine scar secondary to a caesarean delivery. In the absence of endometrial re-epithelialization of the scar area the trophoblast and villous tissue can invade deeply within the myometrium, including its circulation, and reach the surrounding pelvic organs. The cellular changes in the trophoblast observed in PAS are probably secondary to the unusual myometrial environment in which it develops, and not to a primary defect of trophoblast biology leading to excessive invasion of the myometrium. PAS was separated by pathologists into three categories: placenta creta (PC) when the villi simply adhere to the myometrium, placenta increta (PI) when the villi invade the myometrium, and placenta percreta (PP) where the villi invade the full thickness of the myometrium. Several prenatal ultrasound signs of PAS have been reported over the last 35 years, principally the disappearance of the normal utero-placental interface (clear zone), extreme thinning of the underlying myometrium and vascular changes within the placenta (lacunae) and placental bed (hypervascularity). The pathophysiological basis of these signs is due to permanent damage of the uterine wall as far as the serosa, with placental tissue reaching the deep uterine circulation. Adherent and invasive placentation may co-exist in the same placental bed and evolve with advancing gestation. This may explain why no single, or set combination of, ultrasound sign(s) has been found to be specific for the depth of abnormal placentation, and accurate for the differential diagnosis between adherent and invasive placentation. Correlation of pathological and clinical findings with the prenatal imaging is essential to improve screening, diagnosis and management of PAS, and standardised protocols need to be developed.