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How Accurate is Your Pregnancy Due Date?

Updated on September 16, 2008

As soon as you find out you're pregnant, the first thing a health care provider does is attempt to determine your due date. It's pretty convenient to know approximately when your baby will arrive. However, it's important to keep in mind that all women and their babies are different. While we have a pretty good idea, scientifically speaking, of how long it takes until babies are ready to be born, a number of factors come into play.

Photo: Wikipedia
Photo: Wikipedia

Pregnancy Due Date Calculation

There are all kinds of pregnancy due date calculators online, but if you really want to know what's behind it, here's the official calculation:

Date of Last Menstrual Period + 7 Days - 3 Months = Due Date

Example: If you knew your last period started on September 1st it would be

September 1st + 7 Days - 3 Months = September 8th - 3 Months = June 8th

This calculation was created about 1850 when doctors determined the average human gestation was 266 days. Note this is an average. That's why not all women go into labor on their due dates. The more regular your cycle the more accurate your pregnancy due date is likely to be. This is because the calculation was based on the average women having a 28 day cycle and ovulating halfway through on Day 14. Of course, not every woman ovulates on the 14th day of her cycle, has perfect 28 day cycles, or shares the same genetic and lifestyle characteristics.

Unless you regularly track your menstrual periods on a calendar, you may not know the start day of your last menstrual period. This of course adds another unknown variable into the calculation.

Ultrasound performed during the first half of a pregnancy can provide a highly accurate approximation of pregnancy due date. And certain measurements later in pregnancy can give a good idea. But again, nothing is going to be 100% for sure.

Because no one can truly foresee the exact day your baby will be born, it's best not to get too attached to the due date. In the above example, it might be better to think of the due date as early June.

What If I Go Over My Pregnancy Due Date?

Especially for first time mothers, it's perfectly normal to go beyond the projected pregnancy due date. Being uncomfortable during late pregnancy is also very typical. When the due date passes, it tends to aggravate already worn out mothers to be. However, there's nothing to worry about. Those who are still pregnant past the due date can rest assured the baby is coming soon. Most babies are born within two weeks of their due date. If your pregnancy goes beyond 42 weeks, most doctors will consider an induction at that time, if not before.


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  • pregnancy test profile image

    pregnancy test 

    9 years ago

    Really great information, very useful, thanks!


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