If you have your last menstrual period on the 11th and your normal menstrual cycle is 30 days, you can get pregnant even if you have sex on the 20th. Theoretically, the menstrual cycle is fixed, 30 days a time, the day of ovulation is usually about 14 days before the next menstrual period, 4 to 5 days before and after the day of ovulation are considered to be easy to get pregnant; but in fact, even if the menstrual cycle is fixed, ovulation is not set in stone, subject to the influence of emotions, the external environment may be ahead of time or staggered ovulation, or even ovulation twice in January, so anytime you have sex there is a possibility of getting pregnant. Conception is a more complex physiological process, if you have to prepare for pregnancy, it is recommended to use ultrasound, ovulation test paper to monitor ovulation, and reasonably arrange sex to improve the probability of pregnancy; no preparation for pregnancy should use condoms and other measures of contraception to prevent unwanted pregnancy.