The most common reason for brown blood in the fourth month of pregnancy is that it occurs during a miscarriage. After this occurs, you should pay attention to the size of the bleeding, and you should also pay attention to see if you have stomach pain. If the bleeding is not heavy, try to rest in bed and observe. If the bleeding is heavy and there is pain in the stomach, you should go to the hospital in time to prevent miscarriage. During this period, you should also pay attention to keep your mood relaxed and not to have too much stress. If you are under too much stress or nervousness, it will easily lead to an increase in the chance of miscarriage, resulting in unsuccessful birth control. The first four months of pregnancy is a risky period, in the middle of pregnancy, the fetus is basically stable and the chance of miscarriage is not particularly high.