Most of the time, the cause is unknown.
Clefts can run in families and be associated with other problems.
Some drugs taken during pregnancy are known to cause clefts. Clefts are more common in certain ethnic groups. In most cases, however, there is no identifiable cause or risk factor. In the United States, clefts occur once in every 700 to 1000 births, making it a common birth defect. Most children with clefts do not have other birth defects.