Kagami‐Ogata syndrome (KOS14) is a rare disease characterized by omphalocele, polyhydramnios and a bell‐shaped thorax. Although the coat‐hanger appearance of the ribs on postnatal X‐rays is a key diagnostic finding of KOS14, its prenatal diagnosis remains challenging. We encountered a case of KOS14 diagnosed prenatally that showed omphalocele, polyhydramnios, and a bell‐shaped narrow thorax. The coat‐hanger angle (CHA) measured at the sixth thoracic vertebrae and the ribs using three‐dimensional (3D) ultrasonography was 39°, reflecting the coat‐hanger appearance of the ribs. Segmental uniparental disomy chromosome 14 (UPD(14)pat) was confirmed by a methylation analysis and microsatellite analysis after birth. The median CHA (minimum, maximum) in 25 normal fetuses was 19 (9, 26) degrees, and a sonographic CHA of 30° may be a border value for diagnosing KOS14. When the combination of omphalocele and polyhydramnios is found prenatally, 3D ultrasonography for CHA might aid in the differential diagnosis of KOS14.