Cleft lip. Image 2 of 2. Artwork of the face of a person with a cleft, or hare, lip following surgery to correct it. A cleft lip is a birth defect caused by a failure of the face to fuse together correctly during the embryonic stage of development. It can be surgically corrected and there are generally no complications, although dental management and speech therapy may be necessary in some cases. See M350/287 for an image of the person before surgery.