The normal head circumference is the length of one week from the “root of the nose on the forehead” to the “occipital ridge at the back of the head”, and the normal range of head circumference at 23 weeks of pregnancy is 187-209mm. The measurement of the fetal head circumference can help to assess the size of the fetal head, predict the development of the fetus, and help to choose the best delivery method when the mother is in labor. At the same time, when measuring head circumference, some fetuses may be found to have a small or large head circumference.1. Small head circumference: Common causes of small fetal head circumference include genetic factors, poor nutrition during pregnancy, maternal smoking and alcohol abuse, etc. At this time, you need to go to the hospital and have a timely examination. If the fetus is not small, only the head circumference is small, generally do not need to deal with, pregnant women can increase nutrition to promote the growth of the fetus; but if the fetus dual parietal diameter and head circumference are small, then according to the specific circumstances of the pregnant woman and the fetus, under the guidance of the doctor to deal with; 2, head circumference is too large: at present, because the nutrition of pregnant women during pregnancy is generally better, so the fetal head is not uncommon. If the fetal head is simply too large, there is no need to worry. If the pregnant woman wants to have a normal delivery, she should pay attention to diet control and appropriate activities to avoid further increase of the fetal head; if the head circumference is too large due to hydrocephalus, it should be treated in time under the guidance of a doctor, and in severe cases, it may be necessary to induce labor.