How to Calculate One's Conception Date From the Due Date
If you don't know when your last menstrual period was, you may find it difficult to determine the conception date for your child. However, if you have a due date that you are confident is accurate, it is possible to determine a range of conception dates from the due date. It is important to remember that pregnancy lasts roughly 280 days, or 40 weeks. However, since this is calculated from your last menstrual period and not the conception date, day one of your pregnancy actually occurs before the conception date.
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Instructions
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The Steps
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1
Add three months to your due date. For example, if your due date is Sept. 1, add three months, which brings you to Dec. 1.
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2
Subtract seven days. Using the Dec. 1 date from above, that would bring the date to Nov. 24. This is the estimated date of your last menstrual period.
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3
Figure out the range of ovulation dates by adding 14 to 21 days to the estimated date of your last menstrual period. This brings the range of dates from Dec. 7 to Dec. 14. Your conception date will usually be a date within that week.
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Tips & Warnings
Because menstrual cycle lengths vary from person to person, these dates can be off by as much as two to four weeks. Remember that you may have to change the year date when doing this. In the example used, the due date would actually be in the year ahead of the conception date.
