Pyknoachondrogenesis is a lethal skeletal osteochondrodysplasia characterized by severe generalized osteosclerosis. The disease is very rare and only five cases (four males and one female) have been reported in the literature so far. Pyknoachondrogenesis may be detected prenatally due to the extreme shortening of the limbs and hydrops fetalis, or is recognized at birth. The main clinical manifestations include a large head, palpebral edema, a flat nose, low-set ears, a short neck, a short and wide trunk, a prominent abdomen, and severe micromelic dwarfism. Etiology remains unknown. Pyknoachondrogenesis has a lethal outcome, either prenatally or during the early neonatal period. [from SNOMEDCT_US]