The definition of fetal head enlargement. Fetal head enlargement is different from other malformations and cannot be divided directly by numbers. Medically, fetal head circumference refers to the length of the maximum circumference of the fetal head. If it exceeds about twice the average, it may attract the attention of doctors. According to statistics, the average head circumference of most newborns is between 32-36cm. If the fetal head circumference is outside this range, it can usually be judged as having a large head. Possible causes of fetal head enlargement