Mayo Clinic notes that a cleft lip occurs when the tissues which form the lip fail to fuse or fuse only partially, leaving an opening in the lip. The fusion occurs in the second to third months of pregnancy. Scientists attribute cleft lip formation to both environmental and genetic factors.
Mayo Clinic points out that a cleft lip commonly occurs in women who drink alcohol, smoke cigarettes or take certain drugs while pregnant. The drugs which are associated with a cleft lip include anticonvulsants, methotrexate and acne drugs which contain Accutane. WebMD reveals that a cleft lip can occur with a cleft palate, but they develop separately. A cleft lip can occur when there isn’t enough tissue in the lip area of a fetus. It occurs on one or both sides of the mouth.