The method of calculating the date of conception by ultrasound is generally based on the size of the fetus to calculate the number of days of pregnancy, and then reverse the date of conception. In most cases, the date of conception is calculated from the first day of the last menstrual period. If you do not remember the exact date of the last menstrual period, you can also use ultrasound to calculate the date of conception, and the difference between the ultrasound result and that of the menstrual period will not be too different. When doing ultrasound, the doctor will determine the growth of the fetus according to the ultrasound, such as the size of the gestational sac, the length of the top hip, etc. to calculate the number of days of pregnancy, but the doctor will have different diagnostic criteria for different pregnancy periods when doing ultrasound, and the early pregnancy ultrasound or NT ultrasound is usually the standard for estimation. Ultrasound can be used to calculate the day of conception. Ultrasound can be performed at 5 or 6 weeks of pregnancy, and the examination at this time is mainly to diagnose whether there is ectopic pregnancy and other abnormalities.