What causes a hairlip?

A hairlip, commonly known as a cleft lip, is caused by a failure of the tissue that forms the lip to fully come together during early fetal development. This can result from a combination of genetic factors, environmental influences, and nutritional deficiencies during pregnancy. Factors such as maternal smoking, Diabetes, or certain medications may also increase the risk. Cleft lips can occur in isolation or in conjunction with a cleft palate.