Microstomia is a small mouth (micro- a combining form meaning small + -stomia a combining form meaning mouth = (abnormally) "small mouth" in Greek.)
Congenital
It is a feature of many craniofacial syndromes, including Freeman–Sheldon syndrome and Sheldon-Hall syndromes (or distal arthrogryposis multiplex congenita). It may present with whistling-face feature, as well, as in Freeman-Sheldon syndrome. [Source: Wikipedia 
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