The depression at the back of a baby’s head is the posterior fontanel, the part of the skull that does not close. Newborns are born with two fontanelles: the anterior fontanel, which is at the top of the head, and the posterior, which is now in the occipital region, and the posterior fontanel usually closes about three months after birth. In addition, the skull suture of the child is not closed, the child’s head circumference will gradually increase with the development of the brain, the head circumference at birth is usually 32-34 centimeters, will reach 46 centimeters at the age of one year, the fontanel closure time is usually in 12-18 months, the individual not later than two weeks of age. Premature closure of the fontanel is associated with poor brain development. In some children, the fontanel closes at 5-6 months of age, resulting in microcephaly and often more pronounced backwardness in intellectual and motor development. Delayed closure of the fontanel is commonly associated with vitamin D deficiency rickets, which is often referred to as low calcium, so full-term infants should be given daily vitamin D supplements starting two weeks after birth, and more sunlight in the summer.