Etiology

The upper lip is derived from medial nasal and maxillary processes. Failure of merging between the medial nasal and maxillary processes at 5 weeks' gestation, on one or both sides, results in cleft lip.

Cleft lip usually occurs at the junction between the central and lateral parts of the upper lip on either side. The cleft may affect only the upper lip, or it may extend more deeply into the maxilla and the primary palate. (Cleft of the primary palate includes cleft lip and cleft of the alveolus.) If the fusion of palatal shelves is impaired also, the cleft lip is accompanied by cleft palate, forming the cleft lip and palate abnormality.