The fontanel of a five-month-old baby is small, and if the head circumference develops normally, it can usually be observed and followed up. However, if the development of head circumference is abnormal, we need to be alert to the possibility of microcephaly, and improve the cranial nuclear magnetic examination if necessary. If there is intracranial hypertension, it can be treated surgically. The size of fontanel varies greatly from one individual to another. If a five-month-old baby has a small fontanelle, but the head circumference is normal, parents usually do not need to worry too much. A small fontanelle may be due to the baby’s individual development. If a five-month-old baby has a small fontanelle and a head circumference that is more than two standard deviations below the average for children of the same age, the possibility of microcephaly needs to be considered, and further examination in the hospital is required. Cranial magnetic resonance imaging (MRI) can be performed to clarify brain development. The size of the fontanel varies greatly from individual to individual, and brain development cannot be judged solely on the basis of the size of the fontanel. If a five-month-old baby has a small fontanelle and abnormal head circumference, he or she needs to seek medical attention, and if there is intracranial hypertension, he or she can be treated surgically if necessary.