The etiology of birthmark formation may be due to melanin increase, vascular abnormality, heredity or sun exposure, etc. Different types of birthmarks have different pathogenesis. 1. Melaninization: Mongolian spots and nevus of Ota are mainly caused by melanin staying in the dermis during embryonic development. 2. Abnormal blood vessels: Frogfish spots are caused by the aggregation of capillaries near the skin. 3. Heredity: cafe au lait spots are related to heredity, which is a sign of systemic lesions, and multiple cafe au lait spots may be combined with neurofibroma. 4. Sun exposure: pigmented nevus is caused by both heredity and environment, and sun exposure is one of the most important factors that induce and aggravate it. There are many types of fetal birthmarks with different causes, so it is recommended to consult the relevant departments to find out the causes according to different types of birthmarks.