If the menstrual cycle is 25-27 days a cycle, ovulation may occur on the thirteenth day of menstruation, so the day of menstruation on which ovulation occurs depends mainly on the length of the menstrual cycle. Ovulation occurs 14 days before the next menstrual period. If the menstrual cycle lasts 25-27 days, ovulation occurs on the thirteenth day of the menstrual period, so it is recommended that if you are planning to get pregnant, you can choose to have sex at this time, which will increase your chances of getting pregnant. On the contrary, if there is no plan to get pregnant, you can avoid having intercourse at this stage, so as to avoid the possibility of pregnancy. In addition, if the usual menstrual cycle is once every 28-30 days, and if you are affected by emotions in your daily life and by the environment, which leads to an early menstrual period in that month, ovulation may also occur on the thirteenth day of the menstrual period.