The menstrual period is menstruation, and about five days after menstruation may be the safe period or the dangerous period. 1. Safe period: under normal circumstances of the menstrual cycle, the fourteenth day before menstruation is the day of ovulation, the first five days of the day of ovulation, and the next four days are all belong to the dangerous period. If the fifth day after menstruation is clean and not yet in the range of the danger period, it is the safe period, but the safe period is not absolutely accurate. 2. Dangerous period: If the menstrual cycle is relatively short, the fifth day after the menstrual period has reached the dangerous period range. There is no absolute safe period for women, due to environmental changes, heavy work pressure, emotional upset, sexual excitement, etc., may lead to early ovulation. After the menstrual period has gone about 5 days after having sex, once the menstrual period is delayed need to go to the hospital in a timely manner, by the doctor to clearly diagnose whether the pregnancy.