Widening of the extracerebral space and thinning of the corpus callosum in a 4.5-month-old infant may be a physiological phenomenon, or it may be a pathological manifestation of cerebral developmental delay, hypoxic-ischemic encephalopathy, chromosomal abnormality, and so on. 1. Physiological reasons: infants from 2 to 12 months of age will have the symptom of widening of the extracerebral space due to developmental factors, and the symptom is especially obvious from 2 to 6 months of age. Therefore, it is suggested that patients should first observe the condition of their infants, and then review the cranial MRI once every six months to evaluate the changes of the extracerebral space. 2. Pathological causes: such as cerebral developmental delay, hypoxic-ischemic encephalopathy, chromosomal abnormality and other pathological factors can cause infants to have widening of the extracerebral space and thinning of the corpus callosum, so the family members should go to the hospital as soon as possible to assess whether the patient has ataxia, backward intellectual development and other related clinical manifestations. It is recommended to consult a pediatrician to evaluate the child’s condition, find the cause of the disease and receive timely targeted treatment.