Generally, menstruation stops when a woman becomes pregnant. This is because after the fertilized egg is laid, the level of estrogen in the body increases, thus causing no menstruation to occur throughout the pregnancy period. In some women, because the fertilized egg is not stable, the placenta is not formed, and near or at the onset of menstruation, there are still mature eggs that are discharged and bleed out of the body as the uterine lining is shed. Until after the third trimester, when the placenta is formed and estrogen and progesterone are maintained at a high level, menstruation will no longer occur. However, pregnant women should not be too alarmed, as long as the fetus and mother’s health are not affected, no special treatment can be done, and the menstrual changes can be observed at this time. In addition, it should be noted that vaginal bleeding is not always menstrual, but can also be caused by preterm miscarriage or pregnancy combined with cervical polyps, cervical erosion, or even cervical cancer. If abnormal bleeding occurs during pregnancy, it is important to visit a hospital promptly to rule out other causes. If the vaginal bleeding is caused by pathological factors, timely treatment is needed to decide whether to continue to keep the pregnancy or terminate it according to the condition of the pregnant woman. In conclusion, most bleeding during pregnancy is a sign of abnormality, so you should pay attention to it and be alert to it in time and go to a regular hospital to see a professional doctor.