The obstetrician calculates the due date based on the time of the last menstrual period of the pregnant woman. In a 40-week pregnancy, the first day of redness is the first day of pregnancy. However, this 40 weeks is calculated on the basis that the pregnant woman’s menstrual cycle is 28 days, which means that she ovulates on the 14th day of her menstrual flow. Therefore, for women with prolonged menstrual cycles, the calculation of the expected date of delivery is biased and the date of delivery based on the date of the last menstrual period is earlier than the actual expected date of delivery. For this group of women, the calculation should be adjusted in relation to the number of days of the menstrual cycle. For IVF and IUI pregnancies, the exact date of delivery can be calculated because the exact time of ovulation is available. Generally, the day of egg retrieval is the 14th day of your last menstrual period, from which the exact date of your last menstrual period can be calculated 14 days backwards. For IUI cycles, the day of ovulation is used as the 14th day of your last menstrual period to work backwards. In some cases, if the embryos are transferred on day 2 or 3, the day of transfer will be day 16 or 17 of the last menstrual period, while in the case of frozen blastocysts, the day of transfer will be day 19 of the last menstrual period. Precise calculation of the last menstrual period and the expected date of delivery is very important to ensure the safety of both the pregnant woman and the fetus.