Swollen feet can occur in about 75% of pregnant mothers in the late stages of pregnancy, after 28 weeks. However, the fourth month of pregnancy has only just entered the middle of pregnancy, and it is not normal to have swollen feet at this stage. There are two main reasons why physiological edema occurs during pregnancy: First, the uterus becomes larger after pregnancy, which compresses the inferior vena cava and makes the blood flow back poorly. Second, the secretion of estrogen and aldosterone increases after pregnancy, resulting in the retention of sodium and water in the body. Edema occurs significantly earlier during pregnancy, and the vast majority of cases are caused by diseases, the common diseases are mainly hypertensive diseases in pregnancy, kidney diseases, cardiac insufficiency and endocrine diseases. Therefore, pregnant mothers who have swollen feet in the fourth month of pregnancy should not take it lightly and must go to the hospital as soon as possible to seek further medical attention.