The head circumference of a 6-month-old child is approximately 36-39 cm. If the child’s head circumference is within this range at 6 months of age, this is normal. If a child’s head circumference is significantly smaller or larger than this range, it is important to look for the cause. Some pathological factors can cause abnormal head circumference in children, such as hydrocephalus and brain tumors, which can cause a significant increase in head circumference. If a child suffers from vitamin D deficiency rickets, it can cause square cranium, which can also result in a significantly larger head circumference, requiring timely intervention and treatment, such as surgery for hydrocephalus.