Placenta accreta refers to different grades of abnormal placental attachment to the uterine wall, which are characterised by invasion of trophoblast into the myometrium. Placenta accreta has only been described and studied by pathologists for less than a century. The fact that the first detailed description of a placenta accreta happened within a couple of decades of major changes in the caesarean surgical techniques is highly suggestive of a direct relationship between prior uterine surgery and abnormal placenta adherence. Several concepts have been proposed to explain the abnormal placentation in placenta accreta including a primary defect of the trophoblast function, a secondary basalis defect due to a failure of normal decidualization and more recently an abnormal vascularisation and tissue oxygenation of the scar area. The vast majority of placenta accreta are found in women presenting with a previous history of caesarean section and a placenta praevia. Recent epidemiological studies have also found that the strongest risk factor for placenta praevia is a prior caesarean section suggesting that a failure of decidualization in the area of a previous uterine scar can have an impact on both implantation and placentation. Ultrasound studies of uterine caesarean section scar have shown that large and deep myometrial defects are often associated with absence of re-epithelialisation of the scar area. These findings support the concept of a primary deciduo-myometrium defect in placenta accreta, exposing the myometrium and its vasculature below the junctional zone to the migrating trophoblast. The loss of this normal plane of cleavage and the excessive vascular remodelling of the radial and arcuate arteries can explain the in-vivo findings and the clinical consequence of placenta accreta. Overall these data support the concept that abnormal decidualization and trophoblastic changes of the placental bed in placenta accreta are secondary to the uterine scar and thus entirely iatrogenic.
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