If you have your period on the 21st and your menstrual cycle is usually 30 days, the time of ovulation is on the 7th, 4 days before and 3 days after the 7th is the ovulation period, that is, from the 3rd to the 10th is a total of 8 days. If you are planning to get pregnant, it is recommended to arrange your sex life reasonably during the ovulation period, and have intercourse 3 times within 8 days of ovulation, and the 3 times are spaced apart, the chance of pregnancy will be greatly increased. If you are not planning to get pregnant, it is recommended to have intercourse during the non-ovulatory period so as to avoid the possibility of pregnancy. If you have intercourse during ovulation, it is best to use a condom for the entire duration of your sex life to prevent pregnancy.