Routine prenatal ultrasound of a massively obese mother at 21 weeks of gestation revealed short-limb dwarfism in the fetus. The proportionate shortening of tubular bones of about 50 per cent of the normal length, the absence of thoracic dysplasia, and a normal head circumference narrowed the diagnosis down to a severe but non-lethal skeletal dysplasia. Ulnar deviation of the hands and talipes made diastrophic dysplasia the most likely differential diagnosis. At post-mortem clinical examination, the diagnosis of diastrophic dysplasia was clearly apparent due to highly specific 'hitch-hiker thumbs', similarly luxated big toes, facial dysmorphism, and a cleft palate. Retrospective re-evaluation of the prenatal ultrasound videos revealed the misplaced thumbs, which together with the ulnar deviation of the wrist and suspected talipes, led to the conclusion that the definitive diagnosis can be established prenatally, even in a mother with massive obesity.