Macrostomia is a congenital defect resulting from persistent lateral facial clefts, caused by failure of the maxillary and mandibular portions of the first branchial arch to unite normally. Macrostomia is a rare anomaly, with an estimated incidence of 1 in 150,000 to 300,000 births and is most often associated with other anomalies. Unilateral macrostomia is more common than bilateral (summary by Hawkins et al., 1973). [from
OMIM]