The reason for the darkness under the nose of a newborn baby is to consider that it may be caused by factors such as birthmarks or hemangiomas. 1. Birthmarks: Birthmarks are pigmented lesions that appear on the surface of the skin due to abnormal proliferation of human skin tissues during the developmental period. Some of the birthmarks will fade away with age, but some of them can stay with you for life. 2. Hemangioma: Hemangioma is a common vascular tumor disease in clinic, the exact etiology and pathogenesis are not very clear at present, and it may be related to the abnormal proliferation of blood vessels during embryonic development. Benign hemangioma has a tendency to heal by itself, and its prognosis is generally good; malignant hemangioma has a relatively poor prognosis even after treatment. Parents are advised to bring their newborns to the doctor in time to find out the causes and eliminate the triggering factors as soon as possible.