How to calculate the due date

Patient's question:

First fetal movement was on December 18th, please ask which day next year is the due date.
Cannot remember the last menstrual period

Doctor's answer:

Pregnancy Due Date Calculator
Since it is difficult for every pregnant woman to accurately determine the time of conception, medical practice specifies that the due date should be calculated from the first day of the last menstrual period. The entire pregnancy lasts for 280 days, equivalent to 10 gestational months (each gestational month being 28 days). The month of the last menstrual period date plus 9 or minus 3 gives the due date month number; the day plus 7 gives the due date day.
Pregnancy Due Date Calculator 1 calculates the due date based on the last menstrual period: The month of the last menstrual period date plus 9 or minus 3 gives the due date month number; the day plus 7 gives the due date day. If your last menstrual period was on March 1, then your due date would be: March + 9 = December; 1 + 7 = 8th, which is December 8, 2010.