For women with regular menstrual cycles, the day of ovulation is 14 days before the next menstrual period, and 3-4 days before and after the day of ovulation is the ovulation period, except for the ovulation period and the menstrual period as the safe period. Sex during ovulation has a higher chance of pregnancy, while sex during the safe period has a lower chance of pregnancy. However, the safe period is not necessarily safe, and is only suitable for people who have normal menstrual cycles and can correctly predict the safe period. For patients with irregular menstrual cycles, or menstrual cycles that are not 28 days long, their ovulation days may be delayed or advanced. Their ovulation is easily affected by various factors such as external environment, climate, emotion and health status, etc. Therefore, it is not recommended to use safe period as contraception. In conclusion, safe period is a relative concept, which may vary from woman to woman. It is recommended to use condom, contraceptive pill, ligation or ring for contraception, which is more reliable in terms of contraceptive effect.