Pregnancy does not lead to menstruation. If the menstrual cycle is 28 days, ovulation occurs on the 14th day of menstruation. The fertilized egg is formed, divides and proliferates and develops, and is planted in the uterine cavity 7-8 days after ovulation, and produces adult chorionic gonadotropin, which stimulates the ovarian corpus luteum to develop into the corpus luteum of pregnancy and continues to secrete estrogen and progesterone, at which time the endometrium is maintained by estrogen and progesterone and does not shed At this time, the endometrium is maintained by estrogen and progesterone and does not shed blood, so that menstruation does not occur during pregnancy. Vaginal bleeding during pregnancy may not be menstrual, but may be a sign of preterm miscarriage, i.e. a small amount of vaginal bleeding, mostly dark red or bloody leucorrhea, without pregnancy material, before 28 weeks of pregnancy, followed by paroxysmal lower abdominal pain or lower back pain. This condition requires prompt medical attention to avoid miscarriage. In addition, vaginal bleeding can also occur in ectopic pregnancies, mostly around the second month of pregnancy, with accompanying symptoms such as abdominal pain and nausea. Pregnant women with vaginal bleeding are advised to go to the hospital for a timely checkup to determine the development of the gestational sac and the fetal heart and germ. Blood tests for progesterone and chorionic gonadotropin should be performed to determine the cause of the bleeding and to provide targeted treatment.