What is the relationship between the safe period and the menstrual cycle?

Generally, the safe period refers to the relatively infertile phase calculated based on the ovulation patterns within the menstrual cycle, and its estimation directly depends on the regularity of the menstrual cycle. The detailed explanation is as follows:

A normal menstrual cycle ranges from 21 to 35 days, with an average of 28 days. Ovulation typically occurs about 14 days before the next menstrual period. The ovulation period includes the ovulation day itself, the five days preceding it, and the four days following it—ten days in total. The remaining time is considered the safe period. For example, if the menstrual cycle is 28 days and menstruation lasts for 5 days, the time from the end of menstruation to before ovulation, and from after ovulation to the next menstrual period, would be considered safe periods.

However, it should be noted that the safe period is not absolutely safe, as its accuracy highly depends on the regularity of the menstrual cycle. If the cycle is irregular—for instance, if menstruation occurs seven or more days earlier or later than expected—the timing of ovulation may vary, making the calculation of the safe period unreliable. Even with a regular cycle, ovulation may be advanced or delayed due to emotional factors, stress, environmental changes, etc., which can render the safe period unsafe.

Therefore, the safe period refers to the time remaining after excluding the ovulation period and the menstrual period from the menstrual cycle. Its reliability is determined by the regularity of the menstrual cycle. Due to the susceptibility of ovulation to interference, the failure rate of contraception during the safe period is relatively high. It is not recommended as a primary contraceptive method. If contraception is needed, more reliable methods such as condoms or short-acting contraceptive pills are advised.