The causes of baby birthmarks are considered to be related to pigmentation, vascular aggregation, vascular malformation, and genetic factors. 1. Hyperpigmentation: For example, birthmarks such as Mongolian spots and nevus of Ota are usually formed when melanocytes derived from the neural crest fail to reach the basal layer of the epidermis and remain in the dermis during embryonic development. 2. Aggregation of blood vessels: For example, salmon spot birthmark, also known as nevus neonatorum, is mainly due to the aggregation of capillaries near the skin. 3. Vascular malformation: for example, bright red nevus birthmark, which belongs to congenital vascular malformation, mainly due to the developmental disorder or developmental abnormality of primitive blood vessels in the embryonic germinal bud. 4. Genetic factors: for example, pigmented nevus, coffee spot and other birthmarks, the reason for the appearance is mainly related to genetic factors. If your baby’s birthmark affects your health, you should further cooperate with your doctor for examination and treatment, and take your baby for regular physical examination.