How is the estimated due date calculated?

Patient's question:

Detailed medical condition and consultation purpose: Hello! The last menstrual period was on November 25, 2005. Can you help calculate the due date?

Doctor's answer:

The medical method for calculating the estimated due date is: starting from the first day of the last menstrual period, add 7 days to the date and add 9 months or subtract 3 months. For example, if your last menstrual period was December 6, 2009, then your estimated due date would be 12 - 3 = 9. 6 + 7 = 14. The estimated due date is September 14, 2010. However, due to different menstrual cycles and varying times of fertilization and implantation of the embryo, the actual delivery date can differ by 1-2 weeks from the estimated due date.

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