A vertical cleft or clefts in the upper lip. This congenital condition, resulting from the faulty fusion of the median nasal process and the lateral maxillary processes, is usually unilateral and on the left side, but may be bilateral. It may involve either the lip or the upper jaw, or both, and often accompanies cleft palate. Nongenetic factors may also be responsible for causing this condition. The incidence of cleft lip is from one in 600 to one in 1250 births.