Having a period when you are pregnant does not necessarily mean you are menstruating, it can be a miscarriage, preterm labor, problems with the placenta and genital damage or inflammation. Normally, estrogen and progesterone are in a constant state of elevation during pregnancy and menstruation or vaginal bleeding does not usually occur. If bleeding occurs during pregnancy you need to be alert to the following conditions. 1. Pre-eclampsia or preterm labor: the placenta is not fully formed in the first trimester of pregnancy, and fluctuating hormone levels are likely to cause some of the uterine lining to peel off and bleed, but this bleeding can’t be called menstruation, and it is a manifestation of preterm miscarriage. If vaginal bleeding occurs in the middle or late stages of pregnancy, accompanied by paroxysmal abdominal pain and vaginal discharge, similar to labor, it is a symptom of preterm labor, all need to go to the hospital immediately. 2. Placenta problem: If the pregnant woman has placenta praevia or placental abruption, there will be vaginal bleeding, accompanied by back pain, abdominal pain, etc. If the bleeding is caused by placenta problem, you need to go to the hospital for treatment. 3. Genital injury or inflammation: If the pregnant woman has inflammation in the vagina and cervix, or if she has vaginal examination and sexual intercourse, she may have a small amount of vaginal bleeding without abdominal pain. Apart from the above reasons, there are other causes that can lead to bleeding during pregnancy. Regardless of which causes bleeding during pregnancy, you should go to the hospital for a detailed examination in time to determine the cause of the disease and then treat it promptly.