Management of an Advanced Low-Lying Implantation Disorder With Cervico-Isthmic Features at 29 Weeks of Gestation Presenting With PPROM: A Case Report and Literature Review.

Introduction: Cervico-isthmic pregnancy (CIP) is a rare form of low-lying implantation in which the gestational sac implants between the anatomical and histological internal os. It is associated with significant maternal morbidity due to abnormal placentation and a high risk of placenta accreta spectrum (PAS). Advanced cases beyond 20 weeks are exceedingly rare and pose major diagnostic and surgical challenges. Case Presentation: A 30-year-old woman, gravida 2, para 1, with a history of prior cesarean section and Asherman syndrome, presented at 28 weeks and 4 days of gestation with preterm premature rupture of membranes. An ultrasound revealed a viable breech fetus located in the lower uterine segment and cervico-isthmic region. The presence of placental lacunae and bridging vessels suggested PAS. An MRI confirmed significant thinning of the lower uterine segment, nearly complete loss of myometrial tissue, and a preserved endocervical canal, consistent with CIP and PAS. Because of the high risk of hemorrhage, a cesarean section was performed using a posterior uterine incision, followed by immediate hysterectomy due to massive bleeding. Histopathological analysis confirmed the diagnosis of placenta increta in the cervico-isthmic area. Both maternal recovery and neonatal outcomes were favorable. Discussion: This case highlights the diagnostic difficulty of advanced low-lying implantation disorders with features favoring CIP. In late gestation, the anatomic boundaries among CIP, cervical pregnancy, and endogenic/Type I CSP may become difficult to define because the implantation site can expand beyond its original location. Careful evaluation of placental topography using ultrasound and MRI, supported by intraoperative anatomic findings and histopathologic correlation, can help characterize the implantation site and guide diagnostic interpretation. Nevertheless, even with multimodal assessment, overlap between CIP and endogenic/Type I CSP may persist, and definitive classification may remain challenging.