The weight gain of 3-5kg is normal at 21 weeks of pregnancy, but it should be combined with the individual’s physical condition and their own base weight, there will be individual differences. Because of the development of the fetus and the possible increase in food intake during pregnancy, the weight will increase with the growth of pregnancy weeks, generally within the normal range of growth can not be too worried. If the weight gain is too fast, it will increase the risk of huge baby, gestational diabetes, gestational hypertension and other diseases, so we should pay attention to manage it, control the diet, supplemented by appropriate exercise, avoid the intake of food with too many calories and too much sugar, eat less and more meals, eat a balanced diet, and take a walk after meals to help digestion and absorption. If the weight gain is low and far below the normal weight gain standard, problems such as anemia, hypoglycemia and delayed intrauterine development of the fetus may occur, so it is necessary to identify the causes in time, strengthen nutrition, diversify diet, eat more fish, red meat, tofu and other foods with high protein, and supplement with calcium and iron to meet the needs of fetal growth and development. In addition to monitoring the weight change of pregnant women, we should also pay attention to regular ultrasound to monitor the fetal head circumference, abdominal circumference, femur length and other indicators to observe the fetal growth and development to facilitate timely detection of abnormalities.