The due date is calculated on the first day of the menstrual period, but not in all cases. Let’s see how to calculate the due date in different cases. For women with regular menstruation, the due date is calculated on the first day of the last menstruation and the first day of the menstrual bleeding. For example, if the first day of the last menstruation is December 20, the expected date of delivery is September 27, and if the first day of the last menstruation is March 3, the expected date of delivery is December 10. That is, the month of the last menstruation minus 3 or plus 9, the day plus 7. 2, irregular menstruation women irregular menstruation women can not be calculated according to the first day of the last menstruation due date. Why is that? Let’s take an example. Woman A and woman C have different menstrual cycles and last periods. The due date for woman A is 9/17/18, and for woman C, if she also has her last period, the due date is 9/8/18. But in fact, the development of embryos starts from the day of ovulation and fertilization, which means that the gestational age of the fetus in the womb of both pregnant women is the same, and the due date of woman C should be the same as that of woman A. In this case, the due date of woman C cannot be calculated according to her last menstrual period, because she has irregular periods. So how should the due date be calculated for women with irregular periods? If the date of ovulation is clearly defined through ovulation monitoring, the expected date of delivery can be calculated based on the date of ovulation. If ovulation is not monitored and the date of ovulation is not known, this will require a combination of the time of the last menstrual period, the menstrual cycle, the blood hcg value corresponding to the time of menopause, the length of the fetal bud, the time of the appearance of the fetal heartbeat and the length of the fetal head and rump at 11-12 weeks of pregnancy. It is a bit complicated to calculate, but the obstetrician can calculate the due date through these. 3. How do women who have had an embryo transfer calculate their due date? For women with embryo transfer, the last menstrual period is no longer the basis for calculating the due date because of the different ovulation promotion or frozen embryo transfer protocol. In this case, the last menstrual period is calculated based on the time of embryo transfer, and thus the expected date of delivery. For example, the last menstrual period is projected 17 days forward from the date of transfer in the case of oogenesis embryos, or 19 days forward in the case of blastocysts. Again, for example, if a woman E had a cleavage-stage embryo transferred on January 22, her last menstrual period would be January 5 (the day of transfer minus 17 days) and her due date would be October 12 of the following year. 4. For women with regular menstruation, is it more accurate to calculate the due date according to the last menstrual period or according to the date of ovulation? The answer is that it is more accurate to calculate by the date of ovulation. Because we can accurately calculate the gestational age of the fetus from the date of ovulation. 5. Why do we need to calculate a due date? The due date will affect the delivery of the baby. Premature birth, overdue birth, intrauterine growth restriction of the fetus are all relative to the due date. The mode of delivery for gestational diabetes or hypertensive disorders is also related to the due date. Therefore, women who have the requirement to give birth must pay attention to their last menstrual period, menstrual cycle, ovulation date, HCG blood level checked after pregnancy, baby’s fetal heartbeat, fetal movement and other significant events to be a competent mother!