If you have your menstrual period on the 11th, you will not get pregnant on the 20th if you are using birth control. Regular menstrual cycle of women, ovulation occurs in the next menstrual period about 14 days before the onset of ovulation, when the 11th comes to vacation, the day of ovulation for the 24th, and before and after ovulation 4 ~ 5 days for susceptible to pregnancy, so the 20th coitus need to take contraceptive measures, such as condoms, emergency contraceptive pills and so on may not get pregnant, can be under the guidance of a physician to use the drug. For women who do not wish to have children, the reliability of safe period contraception is low, and the side effects of taking emergency contraceptive pills are large, so you can choose safe and effective contraceptive measures under the guidance of a doctor, such as placing an intrauterine device (IUD), taking short-acting contraception, etc., in order to avoid unintended pregnancies.