The actual number of days to conception during early pregnancy can be inferred from the average of the length, width and height of the gestational sac (mm) plus 30. The actual number of days to conception is usually calculated from the first day of the last menstrual period. If the menstrual period is irregular or the last menstrual period is not known, the actual number of days to conception can be inferred from the gestational sac and the diameter of the head and buttocks of the ultrasound during early pregnancy. The average of the length, width and height of the gestational sac (mm) plus 30 is inferred to be the actual number of days of conception, and the diameter of the head and buttocks (cm) plus 6.5 is the actual number of weeks of conception. The last menstrual period is crucial for calculating the actual number of days to conception. Women are advised to remember the time of each menstrual period, and when menstruation is irregular or the last menstrual period is unknown, it is recommended that the ultrasound results of the early pregnancy be preserved for the obstetrician to deduce the actual number of days to conception.