Will a small head affect the intelligence of the fetus? There are two main measurements of the fetal head during pregnancy, one is the head circumference and the other is the biparietal diameter (a) Biparietal diameter: refers to the length of the widest part of the fetal head between the right and left sides. Normal value of fetal biparietal diameter by gestational week (mm) (b) Head circumference: refers to the maximum length of the circumference around the fetal head, which can usually assess the size of the fetal head. Normal values for fetal head circumference by gestational week (mm) In clinical practice, we often encounter pregnant women who are very interested in the ultrasound data, head circumference and biparietal diameter are followed by the corresponding gestational week, and many pregnant women are very concerned about the size of the corresponding gestational week does not match the situation. Head circumference and bi-occipital diameter must be within a certain range, too big is not good, too small is not good. In this article, we will focus on the case of small head, that is, the head circumference and bi-occipital diameter is relatively small. The two tables above have written the normal range of head circumference and bi-occipital diameter, where the mean is the average, +2SD means more than two standard deviations, -2SD means less than two standard deviations. The size of the head circumference and bi-occipital diameter can be slightly inaccurate, but it needs to be kept within plus or minus 2SD. Physiological causes of small fetal head (between the mean and -2SD, not less than -2SD value, generally that is, the gap with the actual gestational week is less than two weeks or less): this situation will not affect fetal intelligence 1, measurement error: ultrasound sometimes there is a measurement error 2, genetic predisposition: parents if the head is small, there may be a fetus with a small head 3, poor nutrition: caused by malnutrition. Pathological causes of small fetal head (less than -2SD value, generally that is, more than two weeks difference from the actual gestational week, and in serious cases, more than 4 weeks difference): this situation needs to carefully find out the causes, to clarify whether there are chromosomal problems, which may affect the intelligence. It is important to focus on the possibility of microcephaly. Microcephaly is a neuronal proliferation disorder with a low incidence and poor prognosis, making prenatal diagnosis of great significance. There are many causes of microcephaly, including genetic factors, infections, perinatal asphyxia, exposure to toxic substances and metabolic disorders. Microcephaly is characterized by a fetal head circumference that is less than 3 standard deviations or more than the average number of normal fetuses in the same gestational week. It has been found that when the fetal head circumference is less than 2-3 standard deviations from the average number of normal fetuses in the same gestational week, and is not accompanied by other intracranial structural abnormalities, the intelligence is normal. Therefore, to diagnose fetal microcephaly, it is recommended to measure the head circumference three times and take the average value, and to observe whether the development of intracranial structures is normal or not, and the development of cerebral sulcus and gyrus, especially the frontal lobe. The intracranial structural abnormalities commonly associated with microcephaly fetuses include hypoplasia of the corpus callosum, penetrating brain injury, total forebrain, hydrocephalus, and cerebellar hypoplasia. Flat forehead due to frontal lobe hypoplasia is another feature of microcephalic fetuses. Flat forehead can also be seen in fetuses with trisomy 21-Trisomy 21, and it is not difficult to differentiate between these two conditions. When encountering a fetus with a small head, the cause should be carefully looked for and clarified in order to analyze it. Genetic factors, infections, perinatal asphyxia, exposure to toxic substances and metabolic disorders are identified, and the treatment is tailored to the different causes.