Below is an instruction from Human. Write a response.
    ### Instruction:
    What causes Hemifacial microsomia ?
    ### Response:
    What causes hemifacial microsomia? For most people with hemifacial microsomia, the cause is unknown. It is believed that something occurs in the early stages of development, such as a disturbance of the blood supply to the first and second branchial arches in the first 6 to 8 weeks of pregnancy.
    