The correct name is cleft lip or hare lip. The causes of cleft lip remain unclear. When the face and skull of a foetus form, the upper lip and palate develop from tissues that lie on either side of the tongue. In normal development, these tissues grow towards each other to meet in the middle and so form a complete palate and upper lip. In cleft lip, it seems that the tissues that form the upper lip grow normally but simply fail to join up in the middle of the face.
It has been suggested that cleft lips are genetic.