If the 31st comes to menstruation, ovulation is 14-20, a total of 8 days, but the premise of this calculation method must be the case of regular menstruation, the cycle is 30 days, in order to accurately calculate the time of ovulation, according to the time of ovulation and then project the ovulation period. If the menstrual period is irregular, it is difficult to calculate the ovulation period. If you clinically want to get pregnant, it is recommended to do ultrasound to monitor ovulation, it is best to start monitoring the follicles through ultrasound on the twelfth day of menstruation, and choose to have intercourse when the follicles have reached the dominant position, so that the chances of getting pregnant will be improved. In addition, you can also use ovulation test paper to monitor ovulation, when the ovulation test paper appears strong positive, 24-48 hours after the appearance of strong positive will ovulate, you can choose to have intercourse.