At birth, babies are born with two bony “skylights” on the top of their heads, medically known as “fontanels. The posterior fontanelle generally closes 3 months after birth, and the anterior fontanelle does not close until 1.5 years old. People often say “skylight” or “fontanel” mainly refers to the front fontanel. The surface of the fontanelle is the scalp, and below it is the meninges, followed by the brain and cerebrospinal fluid. When you put your finger lightly on the fontanelle, you can feel the pulsation. That is the pressure of the cerebral spine changing with the heartbeat and blood pressure, in line with the pulse. When a normal baby sits, the fontanelle is slightly depressed. The cerebrospinal fluid in the skull and the blood and tissue fluid in the body are constantly exchanged and kept in balance. When the body loses more water, the cerebrospinal fluid in the ventricles also decreases, the pressure decreases, and the fontanelle will be significantly sunken. If an infant shows this symptom after vomiting or diarrhea, it means that the body is moderately dehydrated and should be hydrated in time, otherwise, because of the infant’s poor natural regulating ability and lack of tolerance, circulatory failure may occur, which may be life-threatening; extremely thin infants with severe malnutrition caused by improper feeding may also show sunken fontanelle, which requires strengthening nutrition and reasonable feeding. Very few infants have small heads due to maternal infections during embryonic life or poor brain development due to other diseases. The head circumference at birth is small and the fontanelle closes early 5-6 months after birth, resulting in a small head deformity; the head is small and pointed, the forehead is narrow, the nose is collapsed, the lower jaw is small and receding, and there is also mental retardation. In children with cretinism caused by hypothyroidism, the fontanelle will also be closed late; at the same time, there are few and light eyebrows, collapsed nasal bridge, wide distance between the eyes, mental retardation, etc., which can be treated with thyroxine. During infancy, growth is particularly rapid and bone development requires vitamin D and calcium. If not supplemented in time, it is easy to suffer from rickets. After 18 months of life, the fontanelle still does not close, “square cranial” deformity appears, the skull feels like a ping-pong ball shell (cranial softening disease). In addition, it can also form bone bead-like changes, chicken chest, “O” or “X” shaped legs and other deformities. As long as the head circumference is not less than 33-34 cm at birth, not less than 46 cm at 1 week of age, and not less than 47 cm-48 cm at 2 years of age, it is normal and there is no need to worry. Early closure of fontanelle may affect brain development Some children are born with small fontanelle, and when they grow to a few months, the fontanelle is only fingertip size, which many parents think is a good thing. In fact, there is a certain time for the fontanelle to close. The posterior fontanelle generally closes 6-8 weeks after birth, and the anterior fontanelle closes between the ages of 1 and 1.5 years. The size of the fontanelle is closely related to brain development, and if it closes too early, it may be caused by microcephaly or brain dysplasia. If there is premature closure of fontanelle, it should be treated differently. Although the fontanelle closes prematurely, the head circumference continues to grow at a normal rate and generally does not affect the intellectual development; if it belongs to congenital microcephaly, the head circumference is small at birth and some of the fontanelles close before the age of half a year. Early closure of the fontanelle may lead to underdevelopment of the child’s brain tissue, low intelligence, and affect normal development. Therefore, parents should regularly measure their child’s head circumference to understand the child’s neuropsychiatric development. In addition, sunken fontanelle and bulging fontanelle should also be given enough attention. A sunken fontanelle is often caused by dehydration due to severe diarrhea and vomiting, while a bulging fontanelle may be caused by an increase in intracranial pressure, and the child should be checked for encephalitis, meningitis, brain tumors, and other diseases.