If the menstrual cycle is 28-30 days, the menstrual period is calculated according to 7 days, the safe period is 3 days after the menstrual period and 10 days before the next menstrual period, that is, from the 27th to the 7th of the next month and the 17th-27th of the next month is the safe period. Safe period is not absolutely safe, usually affected by many factors, some women ovulation is not fixed, the safe period is just a general range, that is, in this period of coitus the possibility of pregnancy is small, in addition to the time of the safe period, the rest is the danger period, each person’s body hormone levels are not the same, are in a dynamic change, the safe period is just relatively safe. Normal women according to the menstrual cycle to calculate the safe period, the length of the menstrual period is not the same, the length of the safe period is not the same, the menstrual period is also regarded as the safe period, but the menstrual period can not be cohabitated, to pay attention to avoid.