Cleft lip is a treatable birth defect of the mouth that appears as one or more splits (clefts) in the upper lip. Cleft lip can range from a small indentation in the lip (incomplete cleft) to a split in the lip that may extend up into one or both nostrils (complete cleft), and possibly into the palate.

Cleft lip forms early in fetal development. It can be caused from genetic factors or maternal environmental exposures during pregnancy, such as drinking alcohol or using tobacco.

Cleft lip often occurs with cleft palate. These conditions are the most common birth defects of the head and neck. Cleft lip, whether it occurs alone or with cleft palate, is more common in males. Most cases of cleft lip involve only one side of the upper lip and only very rarely occur in the lower lip.

Cleft lip is corrected with surgery, usually within a newborn's first 3 to 6 months. Depending on the type and severity of the deformity, more than one surgery may be needed. Sometimes other treatments, such as speech therapy, are also beneficial. Feeding by bottle or at the breast usually doesn't require any special measures.