A sunken fontanel is usually due to physiologic factors such as water deprivation and the fact that the fontanel has not yet closed. It may also be due to vitamin D deficiency. 1. Physiological factors: The posterior fontanel is not yet fully developed. If the infant has recently been drinking too little water, vomiting, diarrhea, etc., resulting in a high water deficit and a decrease in the amount of cerebrospinal fluid in the ventricles of the brain, the posterior fontanel will be sunken. Normal infant fontanel will be completely closed between 6 to 8 weeks of life, if the baby is born for a shorter period of time, there will also be sunken fontanel, which is a normal physiological phenomenon. 2. Vitamin D deficiency: this disease is mainly due to the lack of vitamin D in infants, commonly found in preterm infants, twins, picky eaters and so on. Children younger than 6 months of age often have softening of the skull, accompanied by waking up at night, excessive sweating, occipital baldness and other symptoms, and some of the children will show a sunken fontanel. In most cases, the sunken fontanel is physiologic and parents should not be overly concerned. However, if the child has symptoms such as waking up at night, parents are advised to take the child to the doctor for examination to determine the cause.