The red spot on the back of the baby’s neck may be nevus erythematosus, localized eczema, skin allergy and so on. 1. Bright red nevus: due to abnormal vascular proliferation and malformation of fetal skin tissues during development, resulting in bright red or dark red patches on the skin. When the baby cries or in hot condition, it will make the local congestion and the red spots more obvious. 2. Localized eczema: As babies lie on the bed for a long time, the skin is stimulated by sweat causing the occurrence of eczema, and localized skin of the child may appear papules, blisters, erythema with itching and other manifestations. 3. Skin allergy: due to the infant contact with allergic substances, such as bedding, clothing and so on. This leads to localized skin allergic reaction, erythema, and accompanied by localized itching and other symptoms, and in severe cases, there may be oozing. When erythema appears on the back of the neck of infants, they should go to the hospital in time and ask the doctor to check and treat it clearly.