A woman’s menstrual cycle can be divided into three phases, the absolute safe period, the safe period and the danger period. The absolute safe period refers to the three days before and the three days after a woman’s menstruation, because during this period a woman’s follicles have not yet begun to develop, so there are no mature eggs to be discharged, and it is relatively safe to have sex. However, there is no absolute safe period for women who have irregular menstruation and a long period of menstruation. A woman’s menstrual cycle can be affected by the environment, health, emotions and other influences, and there will be delayed or early ovulation. Once the menstrual cycle changes, the safe period and ovulation period will also change as a result. It is recommended that women who are sexually active but do not plan to conceive can use other contraceptive measures such as condoms to minimize the occurrence of unwanted pregnancies, as either way of ending a pregnancy can cause great harm to the woman.