In Brief BACKGROUND Alimentary tract duplications are rare congenital lesions that may arise at any location throughout the gastrointestinal tract. The most common site of enteric duplications is the ileum, with only 13% of cases occurring in the colon. CASE Prenatal sonography at 39 weeks' gestation showed a cystic structure with thick walls exhibiting clear peristaltic movements in the right lower fetal abdomen. Inner walls of the cystic structure had small folds suggestive of colonic haustra. The sonographic findings and location of the finding were consistent with duplication of the colon and, possibly, the cecum. Congenital duplication of the cecum was proven at neonatal laparotomy. Ileocecal resection with ileocolonic anatomosis was performed and congenital duplication of the cecum confirmed by pathology examination of the resected specimen. CONCLUSION Unusual sites of alimentary tract duplications detected by prenatal ultrasound include the cecum. A cystic right lower quadrant abdominal mass with peristalsis found at prenatal ultrasonography was confirmed at laparotomy as congenital duplication of the cecum.