Retroperitoneal ectopic pregnancy: A narrative review proposing diagnostic criteria, surgical anatomical...

Retroperitoneal ectopic pregnancy (RPEP) is an exceedingly rare form of trophoblastic implantation within the retroperitoneal compartment, posterior to the parietal peritoneum and anatomically separate from the uterus, fallopian tubes, and ovaries. Proximity to major retroperitoneal vessels, including the abdominal aorta and inferior vena cava, creates potential for severe hemorrhage and complex operative management. Three mechanisms compound this risk by delaying diagnosis: transvaginal ultrasonography is physically limited to the true pelvis and cannot visualize supra-pelvic retroperitoneal implantations; persistent classification as pregnancy of unknown location defers extended imaging; and Zone I implantations may mimic acute pancreatitis, renal colic, or gastrointestinal illness, redirecting assessment away from obstetric consideration. This focused narrative review synthesizes published clinical experience of RPEP to identify recurring diagnostic pitfalls, anatomical patterns, and management themes, and to propose a practical framework for institutional use. The literature search encompassed PubMed/MEDLINE, Embase, Scopus, Google Scholar, and reference lists of relevant publications, from database inception to June 2026, using a structured set of RPEP-specific search terms. No meta-analysis was attempted because the evidence consists of case reports and small case series with heterogeneous reporting. Zone I para-aortic and interaortocaval implantation appears to be the most frequently reported anatomical pattern. Systemic methotrexate alone appears unsuitable as routine definitive therapy for high-risk phenotypes - characterized by elevated beta-hCG, embryonic cardiac activity, or major-vessel proximity. Multidisciplinary operative planning is essential in confirmed or probable cases. Based on this synthesis, we propose diagnostic categories for definite, probable, and possible RPEP; a five-zone surgical anatomical classification intended to support operative planning; and a seven-step institutional escalation framework. These proposals are derived exclusively from very low-certainty evidence consisting predominantly of case reports, small case series, and expert anatomical interpretation. They should be regarded as hypothesis-generating frameworks rather than evidence-based recommendations or validated clinical guidance and require prospective evaluation before clinical adoption.