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  • Kerry C. Bullerdick, M.D.

How do you determine my baby's due date?

A common question many women have in pregnancy is, “When is my baby due?” Knowing your baby’s due date is very important to know how far along your pregnancy has progressed.

With each pregnancy, your baby’s estimated due date will be calculated. Estimates are just that, and a due date does not mean that is definitively when it is expected that your baby will go into labor. In fact, some reports show that only around 5% of babies deliver on their estimated due date with most babies delivering between 3 weeks before the due date and up to 2 weeks after the due date.

Your estimated due date will be calculated based upon your last menstrual period and/or an ultrasound. Both of these methods can over or underestimate the due date of the pregnancy. For example, some women cannot remember their last period, do not have normal periods, or miss periods which can make it difficult to date the pregnancy. Similarly, with ultrasound, the further the pregnancy is along the more difficult it can be to accurately estimate the due date. In situations where the estimated due date by the last menstrual period and the estimated due date given by an ultrasound differ significantly, your due date may be changed to account for these things. Having your due date changed does not mean something is wrong, and is nothing to worry about. Your doctors are just trying to get the most accurate estimate for your baby’s due date.

Photo Credit: Deposit Photos

Please note that the baby’s due date is not determined by the date of the conception, which is often unknown. If you have regular periods, conception usually occurs approximately 2 weeks after the first day of your last menstrual period but the exact date can vary which is why it is not used to calculate the estimated due date.

If you ever have any questions regarding your baby’s estimated due date please do not hesitate to ask your doctor and we will be happy to explain it to you.

You can also learn more about calculating due date by visiting the March of Dimes website​.

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