An extra finger on the thumb refers to polydactyly, the cause of which is currently unknown, and is considered to be related to genetics and the environment. 1. Polydactyly is now mostly considered to be genetically related, belonging to autosomal dominant single gene inheritance, may be located in chromosome 2 or 7. It is often associated with short fingers, syndactyly, and other deformities, and is most common in the thumb and little finger. 2. It may be related to the environment that the pregnant woman is exposed to during pregnancy. For example, during the first 3 months of pregnancy, smoking or using some drugs, chemicals, exposure to some industrial radiation or some rays, etc., may also lead to polydactyly; if combined with gestational diabetes may also lead to polydactyly. When polydactyly is detected, you should go to a regular hospital to assess whether surgical treatment is needed.