The following are the causes of miscarriage in the third month of pregnancy: 1) miscarriage due to malformation of embryonic development or embryonic gene deficiency; 2) miscarriage due to uterine contraction caused by excessive fatigue or sexual stimulation; 3) miscarriage due to trauma; 4) miscarriage due to sudden noise or serious internal diseases. It is important to avoid heavy physical labor and to reduce the frequency of sexual life after pregnancy.