Duodenal atresia concomitant with type-A esophageal atresia (DA + TA-EA) is rare. A pronounced enlargement of a closed loop of the upper gastrointestinal tract serves as an early clue for its prenatal detection. We describe an atypical case of DA + TA-EA in which the dilatation of the upper gastrointestinal tract remained mild. Ultrasonographic examination at 28 weeks of gestation showed mild polyhydramnios. Subsequent detailed sonographic and magnetic resonance imaging studies revealed a mildly enlarged stomach and duodenum that resembled a "double bubble," mild ascites, and polydactyly of the right thumb. Fetal abdominal circumference measurements were within normal range. A female neonate born at 36 weeks gestation did not show abdominal distension. DA + TA-EA was diagnosed based on clinical characteristics and X-ray studies of the neonate; the diagnosis was confirmed by surgery. Duodenoduodenostomy and gastrostomy in the first week of life and esophagoesophagostomy at six months of age were performed with satisfactory results, and the infant developed well. Prominent and/or increasing C-shaped fluid collection in the upper abdomen is a highly useful diagnostic sign for DA + TA-EA, but it is not applicable for all fetuses with this disease. Physicians should bear this caveat in mind to avoid diagnostic delays and initiate prompt postnatal therapy.