If you usually have regular periods, the probability of conception is relatively high if you have intercourse during ovulation, which is 14 days before the next menstrual period. The difference between the time of conception in this case and the time of pregnancy calculated from the last menstrual period is 10-15 days, which is within the normal range. In the case of irregular menstruation, the difference between the time of conception and the time of pregnancy calculated from the last menstrual period is more probable. In this case, the time of conception can be roughly estimated by checking the fetal growth and development indicators through ultrasound. The clinical time of conception is not a fixed value, and it is normal to have an error of 7-10 days in the time of conception calculated by ultrasound, which cannot be projected to a specific day.