Cleft lip. Coloured 3D computed tomography (CT) scan of the skull of a young child with a cleft lip, showing an absence of bone between the nose and mouth. A cleft lip, also called a hare lip, is a congenital (present from birth) defect caused by failure of the face to fuse together correctly during the embryonic stage of development. It affects about 1 in 900 babies. Cleft lip is usually repaired by surgery when the baby is around three months old. There are generally no complications, although dental management and speech therapy may be necessary in some cases.