The 21st day of your period is generally a safe period, but it is still possible to get pregnant if you have unprotected sex.
In general, the normal menstrual cycle lasts 28 days and ovulation occurs about 14 days before menstruation. Fertilization of the egg is possible for 2-3 days after discharge, so if you take the day of ovulation (usually 14 days before menstruation) as the base, the first 5 days and the last 4 days of the period are the danger period, and the rest is the safe period. So the 21st day of menstruation is within the safe period.
Safe period is just that the chances of getting pregnant will be lower, not that you can be completely infertile. Since a woman’s ovulation cycle is also affected by her emotions and other factors, patients with irregular menstrual cycles are usually prone to ovulate early and wrongly. In this way, there will be a failure of safe period contraception.
If you really do not plan to have children, you can use regular use of compounded short-acting contraceptive pills, use condoms, as well as the placement of birth control rings, tubal or vasectomy and other methods of contraception, the effect will be more reliable.