Cesarean Scar Implantation in the Evolution of Placenta Accreta Spectrum: Implications for Recognition and Clinical Management.

Accumulating evidence suggests that cesarean scar implantation represents the earliest manifestation of placenta accreta spectrum (PAS) disorders, reflecting a continuous pathophysiologic process rather than distinct clinical entities. This article examines data supporting cesarean scar pregnancy as a precursor to a substantial proportion of PAS, particularly after cesarean delivery, and advocates for unified clinical approaches to these conditions. The global rise in cesarean deliveries has triggered parallel increases in PAS disorders, characterized by abnormal placental attachment at sites of myometrial scarring where regulatory decidual mechanisms are absent. Histopathologic studies demonstrate that cesarean scar implantation and PAS are often indistinguishable, likely representing different developmental stages of the same condition, with up to 70% of expectantly managed cesarean scar pregnancies progressing to PAS at delivery. First-trimester ultrasound enables early identification of high-risk pregnancies through the use of several cesarean scar pregnancy classification systems, including the crossover sign, which categorizes cesarean scar pregnancies based on the position of the gestational sac relative to the endometrial line. Additional classifications distinguish between "on-the-scar" and "in-the-niche" implantation and implantation position relative to the uterine midline in the transverse plane. These parameters predict PAS severity and outcomes. Despite compelling evidence connecting cesarean scar pregnancy and PAS, most literature focuses on them as separate entities, resulting in fragmented clinical approaches. Here, we propose framing the cesarean scar pregnancy as an early manifestation of PAS. Equipped with an appreciation of the natural history of PAS, we recommend targeted screening for women with prior cesarean delivery, uterine surgery, previous cesarean scar pregnancy, or suspected early pregnancy loss, with critical screening windows at 5-7 and 11-14 weeks of gestation. Early identification and risk stratification enable individualized management decisions through shared decision making to reduce maternal morbidity from unanticipated uterine rupture, hemorrhage, and fertility loss. Recognizing cesarean scar pregnancy as the earliest detectable manifestation of PAS transforms management from reactive to proactive risk mitigation and fertility-sparing approaches, potentially improving outcomes and reducing PAS-associated health care burdens worldwide.