A cleft lip is an upper lip that is split, something that affects one out of every 700 babies in the U.S. Like a cleft palate, it is caused by the failure of the two sides of the face to unite properly while the baby is in the womb. Although no one knows exactly why clefts happen, they have a tendency to run in families. Hormonal imbalances, nutritional deficiencies, and certain drugs when used during pregnancy are possible causes.