Fetal ultrasound can be used in late pregnancy to measure various indicators of fetal growth and development, such as abdominal circumference, biparietal diameter, femur length and so on. Fetus in late pregnancy is small three weeks need to be ruled out first pregnancy week projection error, no projection error may be due to nutritional deficiencies, maternal disease factors, etc., need to be based on the specific reasons to take supplemental nutrition, treatment of related diseases and other ways to deal with. 1. Pregnancy week projection error: especially the irregular menstruation of women, for the last menstrual time forgotten, easy to cause the error of pregnancy week calculation, at this time the fetus may be on the small side. In this case, it is necessary to recheck the week of pregnancy according to the ultrasound in early pregnancy. All the indicators of pregnancy examination are normal, there is no obvious tissue or organ malformation, most do not need to be treated. 2. Nutritional deficiencies resulting in growth retardation: If a woman’s daily diet is poor in late pregnancy, and there is a lack of calcium, protein, zinc, magnesium and other nutrients, it may affect the growth and development of the fetus, resulting in the fetus is small. In this case, it is necessary to increase nutrition, especially high protein, high calcium food, and supplement vitamins and minerals to meet the needs of fetal growth and development. 3. The influence of maternal disease factors: such as pregnant women suffering from hypertension, diabetes, hyperthyroidism, etc., will affect the growth and development of the fetus, resulting in abnormal growth and development indicators. In this case for the mother-to-be’s disease, take the appropriate drug treatment, at the same time to dynamically observe the fetus in all aspects of the situation. Fetus in late pregnancy is small three weeks should first clarify the cause, for the cause of the treatment and treatment, the use of drugs need to follow the doctor’s instructions.