Sonographic evaluation of low-lying gestational sac in the first trimester: A sonographer’s guide to identify caesarean scar ectopic pregnancy

Introduction: Caesarean scar ectopic pregnancy (CSEP) is a potentially serious complication of caesarean section (C-section) which consists of the implantation of the gestational sac (GS) in the hysterotomy scar. All women with a history of C-section are at risk for CSEP. Early diagnosis and treatment are critical to avoid maternal morbidity and mortality. There is overlap in the sonographic appearance of CSEP, low-lying normal pregnancy, cervical ectopic pregnancy and miscarriage in progress. In 2022, a standardised sonographic evaluation and reporting system for CSEP in the first trimester was published to raise awareness of this entity. We aim to provide a practical guide to the sonographer by outlining the pertinent obstetric history, imaging findings and differential diagnoses in these patients. Case Report: We report three cases of CSEP, diagnosed by ultrasonography in the first trimester. We also report three different conditions that may mimic CSEP. We include the clinical details, sonographic findings and patient outcomes. Discussion: The incidence of CSEP is suspected to be under-recognised and thus underreported. CSEP presents a substantial risk for severe maternal complications including uterine rupture, severe hemorrhage and placenta accreta spectrum disorders. This is complicated by challenges in securing an early diagnosis. When a first-trimester ultrasound reveals a GS in the lower third of the uterus, the sonographer must consider the differential diagnoses herein. Conclusion: We will present the recommended criteria using cases demonstrating the sonographic findings. Raising awareness of the four key sonographic markers of CSEP may improve sonographer detection rates, thus facilitating better patient management.