Fetal head circumference at 12 weeks of pregnancy is usually 56mm, and an error of 3-4mm in the test result is within the normal range. Fetal head circumference refers to the maximum length of a week parallel around the head of the fetus. Fetal head circumference can be used to assess the development of the fetus, and can also be used to evaluate the suitability of a normal delivery when a pregnant woman goes into labor. As the fetus develops, the fetal head circumference varies from week to week of pregnancy. Generally, the fetal head circumference is 56mm at 12 weeks of pregnancy, 249mm at 27 weeks of pregnancy, 290mm at 32 weeks of pregnancy, 333mm at 38 weeks of pregnancy, and 339mm at 39 weeks of pregnancy. If the head circumference is too small and does not reach the standard range, it may restrict the development of the fetal brain, and there is a backwardness in physical strength and intelligence. However, pregnant women do not need to worry too much, to the mid-pregnancy timely supplementation of nutrition, the development of the fetus will slowly tend to perfect, to reach the normal range. However, it is necessary to pay attention to a balanced diet, not to eat too much high-fat, high-calorie food, so as to avoid the growth of the fetus is too large, affecting the later delivery, increasing the risk of childbirth.