The harelip is also known as cleft lip, and the development of this disease is related to genetic factors, environmental factors during the mother’s pregnancy, and the mother’s medication during pregnancy. 1. Genetic factors. When parents or relatives have cleft lip and palate, the risk of the fetus developing cleft lip increases, so this disease may be related to genetic factors. 2. Environmental factors at the time of the mother’s pregnancy. When the mother has active smoking, passive smoking, alcohol consumption, excessive exposure to radiation, vitamin deficiencies in the first trimester of pregnancy, and infections with potentially teratogenic viruses or bacteria during pregnancy, the risk of the fetus developing cleft lip is increased. 3. Mother’s medication during pregnancy. If a pregnant woman has diabetes or needs to take medications such as phenytoin sodium or methotrexate for other illnesses, it may also increase the risk of the fetus developing a cleft lip. After the occurrence of cleft lip, it is recommended to seek timely medical treatment to identify the specific cause.