A Different Way to Calculate Your Due Date

Standardizing Calculations

Pregnant woman standing in front of calendar
Jennifer L. Boggs/Getty Images

The American Congress of Obstetricians and Gynecologists (ACOG) has proposed standardizing how due dates are calculated. One of the things pointed out was that, currently, there are many different ways that can be used to calculate a woman’s due date. When this happens there can be variations that can drastically impact the health of the baby.

What is being suggested includes:

  • An ultrasound in the first trimester for the most accurate dating of a pregnancy. High quality is a word that is highlighted in their documentation.
  • Obtaining information about the last normal menstrual period from mom.
  • Using both of these data points to arrive at the estimated due date, but ensuring that they not only discuss this clearly with you but chart it prominently on your medical records.
  • In the case of a pregnancy as a result of using assisted reproductive technology (ART), the gestational age would be established based on the age of the embryo at the date of embryo transfer.

One thing they hope to prevent is the changing of due dates at the last minute. While it might need to occur, it should only be done rarely if this practice is put into place. This can help reduce the number of babies who are born too early because of a failure in communication or misestimation of the due date for other reasons including faulty data.

A scenario where it might wind up being really important can include if your practitioner is not on call and sees your due date and assumes it was arrived at using a method they prefer, and making decisions based on that, when in fact, your due date was arrived at in another way, making your pregnancy not as “old” as believed, which can be harmful to your baby in terms of accidental prematurity.

A couple of things that jump out at me that some people might have questions about:

  1. Will this mean that everyone gets an ultrasound in the first trimester? The short answer appears to be yes. Though not everyone has access to high quality ultrasound, starts prenatal care in the first trimester, despite recommendations, and the differences found in due dates for ultrasound versus women with accurate menstrual/ovulation history are not vastly different (+/- 10 days with ultrasound versus +/- 14 days with LMP).
  1. Who will pay for these added expenses of the ultrasound? Some women will not have health care coverage and the out of pocket expenses for this ultrasound may not be within their means. And with about 50% of births being covered by Medicaid, this will increase costs upfront for those on Medicaid. Hopefully, the saving would be recouped by preventing preterm births on the other end, but the initial expenditure will still need to be realized, particularly for those with no money or poor insurance coverage.


Definition of term pregnancy. Committee Opinion No. 579. American College of Obstetricians and Gynecologists. Obstet Gynecol 2013;122:1139–40.

Method for estimating due date. Committee Opinion No. 611. American College of Obstetricians and Gynecologists. Obstet Gynecol 2014;124:863–6.

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