The possible causes of dyspnea during pregnancy are not exactly the same at different stages of pregnancy: 1. The symptoms of dyspnea that appear in early pregnancy are usually caused by the normal early pregnancy reaction and are related to the elevated hormone levels in the body. 2. The symptoms of dyspnea that appear in mid-pregnancy are usually caused by anemia during pregnancy and the increased burden on the heart due to the increased circulating blood volume during pregnancy. The most common cause of respiratory distress during pregnancy is during the 32nd to 34th week of pregnancy, when the increase in circulating blood volume reaches its highest value and the burden on the heart increases significantly, making it easy for respiratory distress to occur. Moreover, due to the enlargement of the uterus, the rise of the uterine fundus raises the diaphragm upward, which affects the normal activities of the heart and lungs, thus causing the symptoms of dyspnea.