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Anthropology of the Due Date

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So much mysticism and mythology surrounds the pregnancy due date. Much of the mysticism is held by physicians who hold on to the old ways of determining when a pregnant woman will deliver. Believe me; most physicians would love a way to determine the due date so we could plan our lives around the deliveries of our patients, but the truth is only 1 to 2 percent of women will actually deliver on their due date. 


So what determines a due date and what is the difference between EDC (estimated date of confinement) and EDD (estimated date of delivery) and what the heck is Naegele’s Rule? This post will help show the origins of the due date and how we are currently using a system that is about 250 years old.


Franz Karl Naegele (1778-1851) was the German obstetrician who initially came up with the rule to determine a woman’s due date based on her last menstrual period (LMP). There are many ways to calculate Naegele’s Rule. I use the system where you take the LMP, add seven days, and subtract three months. So if your LMP was April 1, 2009 then your due date would be January 8, 2010. You can impress your friends at parties with this maneuver. There are problems with Naegele’s Rule and many people have pointed out that this 250-year-old method is no longer appropriate for our advanced age. What are some of the potential errors with calculating the EDC in this method?


  • It assumes that you are having a regular period and that you ovulate on day fourteen of your cycle. I am a gynecologist and there are many women out there that have irregular cycles that ovulate on day twenty, twenty-five, twelve, fifteen … you get my point. This obviously would add potential error to the EDC determination and could change things by days to weeks.
  • There is another assumption that the routine pregnancy is 280 days long, and that is based on our current calendar system. The problem with this is that there are many months that contain thirty days or thirty-one days and what happens in a leap year, or if you are not pregnant over the shorter month of February. The point is that there is a movement out there that is trying to say that the number should be 288 days and that we are inducing women that have premature babies. A study done in 1990 stated that the proper method for determining a due date was to take the LMP, count back three months and add fifteen days for a primiparous (first pregnancy) woman or ten days for a multiparous (subsequent births) woman. This was published in the journal Obstetrics and Gynecology.
  • There are many that argue this method of calculating the EDC is as archaic as the term EDC itself. Lending to the agrarian societies from whence it came, the EDC literally came from the fact that a woman was confined to her bed for the last part other pregnancy to prevent preterm labor. While we still prescribe bed rest today as a possible therapy for preterm labor, it does seem odd that the medical establishment uses terminology from the 1700s.


The due date is as individual as the pregnant mother is. While the EDC is currently calculated by Naegele’s Rule this does seem a bit archaic and inefficient, especially if we are using this dating method to determine inductions and postdatism. There have been other methods with increased accuracy but they require a woman to measure body temperatures and be move involved in her own self-care. Many reading this article are very involved with birth and feel as though self-care is very important, but there are many women out there that simply choose not to be observant of their own cycle. So, what do we “do” with the “due”. Unfortunately, I think we will keep going with the current system and back it up with ultrasounds which are accurate within 5 days if done in the first trimester.


Approximately 3 percent of so-called term births (occurring after thirty-seven weeks) are completed with fetal lung immaturity and this could be because the baby may have been between thirty-five to thirty-seven weeks and not term.


Are we too involved in the birthing process? Are there better ways to determine the pregnant due date or should we not worry about it and just let babies be born on their own time. The Latin word natura gives rise to the word natural and means “to be born.” Maybe we should just leave well enough alone. 


By Shawn Tassone, MD for Barefoot & Pregnant

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