Cleft lip, or harelip, is a birth defect in which the upper lip is split on one or both sides. It may occur with cleft palate, a partial to complete split of the roof of the mouth. Cleft lip occurs about once in every 1,000 births and is more common in males. Cleft palate, when observed alone, is more common in females and occurs about once in every 2,500 births. The defects result from incomplete fusion of tissue or bone, or both, during fetal development. Early surgery usually can repair the defects, but cleft palate in particular may also be linked with hearing, breathing, or tooth-placement problems. The defects are genetic in origin in some cases, and some rare forms are linked with a defective gene.