cleft lip A fissure in the upper lip that is due to failure of the left and right sides of the fetal lip tissue to fuse, an event that should take place by 35 days of fetal age. Cleft lip can be on one side only or on both sides. Because failure of lip fusion can impair the subsequent closure of the palatal shelves, cleft lip often occurs in association with cleft palate. It is one of the most common physical birth defects, and it can be corrected with surgery.
