Abstract
BACKGROUND: Fetus-in-fetu (FIF) is an extremely rare congenital anomaly characterized by the development of a parasitic fetus within the body of its twin. This condition typically results from abnormal embryogenesis in a monochorionic diamniotic pregnancy. Its incidence is estimated to be approximately 1 in 500,000 live births, with fewer than 200 cases reported in medical literature. CASE PRESENTATION: We report the case of a 21-yr-old pregnant woman at 35 wk and 1 day of gestation, confirmed by first trimester ultrasound. The nuchal translucency screening was delayed due to the patient's late presentation. Her obstetric history included a normal vaginal delivery 2 years prior, resulting in a healthy full-term female infant. Neither the patient nor her spouse reported any significant medical history, genetic disorders, or hereditary diseases. The patient had received routine prenatal care, including folic acid supplementation from conception and one dose of COVID-19 vaccine. She did not receive specialized high-risk obstetric care and was managed with standard prenatal services at a primary healthcare center. Her initial consultation with a gynecologist occurred on October 14, 2024, when the FIF diagnosis was established. All laboratory investigations were within normal limits. CONCLUSION: FIF represents a rare yet clinically significant condition that requires high clinical suspicion for accurate and timely diagnosis. Differentiating FIF from mature cystic teratoma is crucial, as their therapeutic approaches and prognostic outcomes differ substantially. Reporting new cases can enhance our understanding of its pathogenesis and contribute to improved patient outcomes.