In the daily outpatient clinic, there are often patients who hold the blood HCG test results in their hands but still ask the doctor with a doubtful look and tone, “How can I be pregnant when I have just had my period? How can I be pregnant when I’ve just had my period? Expecting the doctor to act like a judge and tell her who to trust and what to do next. Leading a patient out of such a swamp requires familiarizing and recognizing normal menstruation. Regular menstruation marks the maturation of reproductive function. The so-called menstruation is the shedding of the uterine lining and bleeding that accompanies the ovarian cycle, which is cyclical and self-limiting, and cyclicality is manifested in the concept of the menstrual cycle, which is the one that patients are most likely to confuse. From the first day of bleeding is the beginning of the menstrual cycle, the interval between the first day of two menstrual periods is called the menstrual cycle. It usually lasts 21-35 days. Its self-limiting nature is the duration of the menstrual period, which is usually 2-8 days, with an average of 4-6 days. Menstrual blood is characterized as non-clotting, dark red, containing blood, endometrial debris, cervical mucus and detached vaginal epithelial cells; it is non-clotting due to the presence of a large amount of fibrinolytic enzymes, but there are exceptions to this rule, for example, clots may be present in the event of a large amount of blood or a high rate of blood flow. Clinically, the normal menstrual flow is 5-80 ml. Normal menstruation is based on the regulation of the hypothalamus-pituitary-ovary axis and the health of the internal and external reproductive organs such as the uterus and vagina. After pregnancy, physiological amenorrhea occurs due to the influence of a large amount of estrogen and progesterone secreted by the placenta, and vaginal bleeding does not usually occur. Therefore, it is obvious that the vaginal bleeding after pregnancy is not real menstrual blood, and there are hidden risks, such as various types of miscarriages, ectopic pregnancy, uterine cavity anatomical anomalies, uterine fibroids, cervical lesions, hyperemesis gravidarum, and embryo uterine cavity landing position abnormality, and so on. Women who have had a history of cesarean section should be more alert to the possibility of keloid pregnancy. Therefore, women who are sexually active must go to the hospital for blood HCG test to determine whether they are pregnant or not when they notice abnormal vaginal bleeding, and it is recommended that they undergo ultrasound examination under the professional guidance of the doctor at the right time in order to avoid delaying the diagnosis and treatment, which may result in irreparable damage or even the risk of death.