We conducted a descriptive cross-sectional study, at the Regional Medical Imaging Center of Ngaoundéré (CRIMN), Cameroon, evaluating the diagnostic utility of obstetric ultrasound in identifying causes of third-trimester metrorrhagia (vaginal bleeding ≥28 weeks of gestation). From May to October 2021, 30 women with metrorrhagia among 302 obstetric ultrasound referrals were included. The prevalence of third-trimester bleeding was 9.93%, with retroplacental hematoma (30%), intrauterine fetal death (23%), and premature rupture of membranes (20%) as leading causes. Placenta previa accounted for 10%. Notably, 60% of bleeding episodes in viable pregnancies had no identifiable ultrasound etiology—a rate higher than in high-resource settings. Sociodemographic analysis revealed younger women (18–25 years) were disproportionately affected by fetal death and membrane rupture, while placental abruption predominated in older women (26–35 years). Urban residence (63%) suggested potential referral bias, whereas rural representation (37%) highlighted gaps in prenatal care access. The study underscores obstetric ultrasound’s crucial role in diagnosing life-threatening conditions (e.g., abruption, previa) in low-resource settings. However, its limitations in detecting subtle etiologies emphasize the need for complementary diagnostics and improved operator training. These findings advocate for enhanced prenatal monitoring and targeted interventions to reduce maternal-fetal morbidity in sub-Saharan Africa. Strengths included standardized imaging protocols and ethical rigor, while limitations involved sample size constraints and the single-center design.