Cleft lip is a congenital birth defect that causes a gap in the top of the lip. It does not affect the roof of the mouth, called the palate. The gap in the lip may be small, which is known as a partial or an incomplete cleft lip. If the gap is larger and runs up to the nose, it is a complete cleft lip. It may occur on only the right or left side of the lip, which is unilateral, or it may occur on both sides, which is bilateral.

Cleft lip is formed in the first few months in the womb, which is when the face and skull form. The tissues of the lip fail to join together, causing the opening. It may be passed on genetically, or it may be a syndrome associated with other genetic abnormalities. Cleft lip may also be the result of drug or alcohol abuse. In addition, there are many cases where the cause is unknown.

Cleft lip can typically be repaired by surgery if the procedure is done early in the infant's life. It does not usually interfere with a baby's ability to nurse, but it can cause speech problems later in life if it is left untreated.