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Harvard Commentaries
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Harvard Commentaries
Reviewed by the Faculty of Harvard Medical School


Calculating Your Due Date


May 20, 2013

 

Your due date, an estimated date of when the baby will be born, is based on the length of an average pregnancy -- 280 days, or 40 weeks from the first day of your last period.

Using this method of calculation, and assuming that you conceived on the 14th day of 280 days, you weren't officially pregnant during the first two weeks of your pregnancy. You may also be surprised to learn that you're very unlikely to give birth on your exact due date. In fact, the chances of this happening are only 5 percent! You are, however, extremely likely to have your baby within two weeks on either side of this date, because a normal pregnancy can last between 37 and 42 weeks.

A Handy Due Date Formula:

Start with the first day of your last period and count backward three months to get the month in which you will deliver. Add seven to the day of the month in which your last period began to get the due date. For example, if your last period began on August 20, you would count backwards three months to May, then add seven to 20 to get a due date of May 27.

Enter The First Date Of Your Last Period:

Your due date is about

You are in your month

This tool is intended for informational purposes only and is not meant to substitute for a professional medical assessment. Your responses to this tool are not collected in any way.

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