The best time to give birth is calculated as the solar time of the last menstrual period, the number of months minus 3 or plus 9, and the number of days plus 7. 1. Regular menstrual cycle: The cycle is regular and the ovulation period is relatively fixed. The expected date of labor can be estimated by referring to the time of the last menstrual period, starting from the first day of the last menstrual period and working backward for 280 days is the expected date of labor. 2. Irregular menstrual cycle: The expected date of labor calculated from the last menstrual period will deviate greatly. The week of pregnancy can be calculated by referring to the result of ultrasound examination between 7 and 12 weeks, and then the expected date of labor can be calculated. In clinical practice, no matter which method is used to calculate the expected date of labor, it is only an approximate time of delivery. It is normal to give birth at 37-42 weeks after pregnancy, and 38-41 weeks of pregnancy is the optimal period for labor. In addition, pregnant women must follow the requirements of labor and delivery tests during pregnancy.