The causes of small fetus are mainly the following: first, the mother’s nutrient deficiency, such as the mother’s partial diet or the intake of nutrients during pregnancy, such as insufficient protein intake; second, the mother suffers from some pregnancy-related diseases, such as gestational diabetes, gestational hypertension, etc., which may lead to insufficient supply of nutrients to the fetus; third, there may be abnormalities in the development of the fetus itself, such as The fourth factor is the placenta and umbilical cord, such as the umbilical cord is too thin, sail-shaped placenta and other conditions may affect the development of the fetus. If the cause is found, treatment should be given to improve nutrition and placental circulation while actively improving the cause.