The bluish color around the mouth of a three-day-old baby can be caused by physiological reasons such as exposed blood vessels, low temperature, or pathological reasons such as heart or lung diseases. 1. Exposure of blood vessels: the skin around the mouth of a three-day-old baby is relatively thin, and the blood vessels under the skin are easily exposed, and in some cases the blood vessels under the skin can be seen, revealing the bluish color around the mouth. 2. Low temperature: the ambient temperature around the newborn baby is too low, the baby’s thermoregulatory center is not well developed, the ability to adapt to the environment is poor, too cold may appear around the mouth of the blue situation. 3. heart disease: the newborn three days around the mouth of the baby blue may also be congenital heart disease caused. 4. Lung disease: If a three-day-old baby has a lung infection, such as neonatal pneumonia or lung dysplasia, it may also lead to blue around the mouth. There may also be other reasons for the greening around the mouth of a newborn baby, it is recommended to go to the hospital in time to improve the examination to clarify the cause of the disease, and then give targeted treatment or treatment under the guidance of the doctor.