If you have your period on the 4th, and your period is usually regular and comes once a month, you will ovulate on the 20th, and from the 16th to the 23rd, a total of 8 days, is the danger period, which is clinically known as the ovulation period. If you have intercourse during the time of ovulation and do not take contraceptive measures, the chances of getting pregnant will be very high in the clinic, and the time other than this is the safe period. However, it is worth reminding that the calculated safe period is relatively safe if menstruation is usually regular. If menstruation is not regular, it is clinically difficult to accurately calculate the ovulation period, so the use of safe period contraception is not safe, it is recommended that you can use condoms to contracept, so as to achieve the purpose of adequate contraception.