In the clinic, I often hear professors say, “This child has a low forehead and a small head, so he will be mentally abnormal in the future”. It seems to be understandable that a small head will have abnormal intelligence, but to what extent is a small head considered a microcephaly? Does a small head circumference necessarily affect intelligence? What causes microcephaly, and what can we do to prevent and treat it? How can we detect microcephaly early? This is what we want to know today. The head circumference of a person has a different range of normal values with age, and the head circumference is small or not small compared to the range of normal values. The head circumference of a normal full-term child is about 32-34cm at birth, increasing by about 2cm per month from 0 to 3 months, 1cm per month from 4 to 6 months, 0.5cm per month from 6 to 12 months, and 2cm between 1 and 2 years of age, with a head circumference of about 50cm at 5 years of age and 54-58cm at 15 years of age. Boys’ head circumference will be smaller than that of boys of the same age by about 0.5-1cm, and the head circumference of preterm babies will be calculated according to the corrected age. A head circumference smaller than the average of 2.5 cm for children of the same age is considered suspicious microcephaly and less than 5 cm can confirm the diagnosis of microcephaly. Microcephaly in children is often detected during community or hospital physical examinations, but attentive parents can also detect it early on their own. To detect microcephaly in time parents need to learn to measure the head circumference themselves. Head circumference is measured by using a soft ruler to go around the arch of the child’s eyebrows, which are the two eyebrows, and then to the occipital ridge, which is the highest point of the back of the human head, and going around the length of one circle. A small head circumference indirectly indicates the child’s lack of brain capacity, which inevitably affects the child’s potential for intellectual development. If the child reaches the standard of suspicious small head or small head deformity, parents should be very vigilant, do not hold a fluke mentality, must promptly go to the hospital specialist to find the cause and find countermeasures. There are many reasons for small head circumference, roughly divided into congenital and acquired causes. Congenital causes are due to genetic or chromosomal abnormalities that cause the brain to become small and never develop normally, and the consequences are often very serious, while acquired causes are often due to a disease that impairs further brain growth, resulting in a small head. Congenital causes are difficult to prevent and almost impossible to treat, and can only be detected in time through pregnancy tests and consultation with an obstetrician for proper management. In contrast, acquired factors can often be prevented. Reducing the damage and limiting factors during the period of rapid brain development from pregnancy to 1 year old is the best strategy, such as reducing infections in early pregnancy, enhancing nutrition during pregnancy, paying attention to feeding after birth, and timely medical checkups are all effective preventive factors. Among the common factors that cause small head circumference, premature closure of fontanelle caused by excessive calcium supplementation is also a factor that cannot be ignored and should be avoided as much as possible. The treatment of microcephaly varies according to the causes of microcephaly, so parents must find out the tendency of microcephaly in their children in time and consult a pediatric rehabilitation specialist for countermeasures, early intervention and early treatment to prevent serious consequences such as mental retardation and delayed language development in children.