There are several common reasons for having brown blood on the 45th day of pregnancy: first, it may be an abnormal condition of the fetus in the uterus. If the fetus develops abnormally or abnormally in the uterus, the pregnant woman will often experience vaginal bleeding during her pregnancy. This phenomenon is irregular and may occur at any time. The color of the blood can be light or heavy, sometimes with blood clots. Secondly, the mother herself has problems, for example, the mother may have a dysfunction of the coagulation function, or hyperfunction of the procoagulant system, which leads to the phenomenon of blood oozing out of the blood vessels and flowing out of the vagina. In this case, the pregnant woman should check the amount and activity of coagulation factors, platelets and prothrombin, and also see if the anticoagulant system is too hyperfunctional and so on. Thirdly, it may be that the pregnant woman suffers from a congenital disease, such as hemophilia, which can also lead to vaginal bleeding during pregnancy.