Usually, around 4 months of pregnancy, pregnant women will start to gain weight significantly, but there are individual differences. After 4 months of pregnancy, the early pregnancy reaction basically disappears, and the pregnant woman’s appetite recovers or even increases greatly. At the same time, the fetus in the uterus begins to develop rapidly, and the weight of the fetus increases, resulting in the need for pregnant women to consume more nutrients and energy to supply the development of the fetus, which leads to a gradual increase in maternal weight. Weight gain during pregnancy is usually caused by the combined effect of estrogen and progesterone, which affects the distribution of fat in the body and leads to weight gain. Weight gain during pregnancy comes mainly from the uterus and its contents, the breasts, increased blood volume, interstitial fluids, and a small amount of maternal fat and protein stores, with an average increase in maternal weight during pregnancy of 12.5 kg. Maternal weight gain can affect the near- and long-term health of the mother and child. Excessive maternal weight gain can increase the risk of macrosomia, obstructed labor, birth injury, and gestational diabetes mellitus; insufficient maternal weight gain is associated with fetal growth restriction, preterm birth, and low birth weight, so weight management should be emphasized. Pre-pregnancy BMI of pregnant women can be determined during their first obstetric examination, providing personalized guidance on weight gain, diet and exercise for pregnant women. It is recommended to have a reasonable diet during pregnancy, balanced nutrition, eat small meals, forbid overeating, avoid high-fat, high-sugar foods, with aerobic exercise, and pay attention to ensure adequate rest.