It is considered normal to have your period at the beginning and at the end of the month if the time difference is 7 days. The normal menstrual cycle is 21-35 days, with an average of 28 days. It is calculated from the first day of the last menstruation to the first day of the current menstruation, which is considered to be the 1st-5th at the beginning of the month and the 25th-30th at the end of the month. The ovulation period is about the 14th day before the next menstrual period. In this case, a few days after the menstruation is clean, you will soon enter the ovulation period, if you do not want to get pregnant, you should pay attention to contraception. If the menstrual cycle is less than 21 days, it is not normal, and there are many reasons for this: 1. diseases of the uterus, such as endometritis, which can lead to congestion and edema of the endometrium, and in more serious cases, can lead to incomplete menstruation; 2. functional disorder of the uterus, which is a kind of menstrual disorder; 3. cervical cancer, which is a serious disease. The bleeding may be irregular and small at the beginning, and may develop into heavy bleeding at a later stage. 4. It may be a miscarriage, which can cause vaginal bleeding after pregnancy and miscarriage. 5. Excessive emotional anxiety, excessive stress and frequent late nights.