Using the time of intercourse plus 266 days to calculate the expected date of labor is not allowed, is not scientific, because the clinical calculation of the duration of pregnancy is from the first day of the last menstrual period, not from the day of intercourse, from the first day of the last menstrual period to the expected date of labor, the entire duration of pregnancy is 280 days. If you start counting from the day you have intercourse, the pregnancy will be prolonged and the calculated due date will not be accurate. Therefore, you must calculate the due date by knowing your last menstrual period under the guidance of your doctor. If the menstrual cycle is not regular, the due date calculated from the last menstrual period may not be accurate. It is recommended to take the first ultrasound during early pregnancy to know the size of the gestational sac, and then calculate the due date according to the size of the gestational sac, which is relatively more accurate.