The timing of pregnancy in clinical practice is usually impossible to calculate exactly because it is based on the woman’s menstrual cycle and ovulation. If the menstrual cycle is normally normal, ovulation is also normal and is usually calculated from the first day of the last menstrual period. However, for women with irregular menstrual cycles, their ovulation period is also irregular and it is not possible to calculate the date of pregnancy by this method. Therefore, the medical rule is to calculate the date of pregnancy or the expected date of delivery from the first day of the last menstrual period, which is 280 days for the entire pregnancy period and 10 gestational months (each gestational month is 28 days). A pregnancy is considered full term if the woman delivers within 38-42 weeks of gestation. Since the length of the menstrual cycle varies from woman to woman, it is normal for the estimated date of pregnancy or due date to vary by 1-2 weeks from the actual date. Usually, the date of pregnancy is calculated in order to estimate the expected date of delivery, which can usually be calculated based on the last menstrual period, such as the month of the last menstrual period plus 9 or minus 3, which is the number of months of the expected date of delivery, and the number of days plus 7, which is the expected date of delivery. If you can’t remember the date of your last menstrual period, you can calculate the date of fetal movement, which is the day of fetal movement plus 20 weeks for first-time mothers and 22 weeks for menstruating mothers. A more accurate clinical method is the ultrasound examination of the inter-parietal diameter of the fetal head, head and hip length and femur length to estimate the gestational age and project the expected date of delivery.