The size of the gestational sac can calculate the approximate time of conception, but not the actual day of conception, which is not so precise. Women who have regular menstrual periods can generally estimate the approximate number of days of pregnancy and the time of conception based on the time of the last menstrual period, while women who have irregular menstrual periods can estimate the approximate time of conception based on the size of the gestational sac. However, since the development of the gestational sac and the shape of the gestational sac are not exactly the same, it is only possible to estimate the approximate number of days of pregnancy and the approximate time of conception based on the size of the gestational sac, but it is not possible to know exactly what day of the month the conception actually occurred. If a woman wants to know the date of conception, she should combine the time of her last menstrual period and the size of the gestational sac to make a guess, which is relatively more accurate, but not 100% sure which day she conceived.