Cleft lip and cleft palate are birth defects that occur when a babys lip or mouth do not form properly during pregnancy. The exact cause of these conditions is not completely understood, but most scientists believe that they are caused by a combination of genetic and environmental factors. Some children have a cleft lip or cleft palate because of changes in their genes, while others may inherit a gene that makes them more likely to develop a cleft, and then an environmental trigger actually causes the cleft. In some cases, cleft lip and palate can occur as part of a condition that causes a wider range of birth defects, such as 22q11 deletion syndrome and Pierre Robin sequence. Its very unlikely that cleft lip and palate have been caused by anything the mother did or did not do during pregnancy.