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Due Date Myths and the concept of the Due Month

Planner with pen and month july when planning baby's due date

Without a doubt, the minute you announced your pregnancy, the first question everyone asked was, "When are you due".

In years gone by the answer used to be something like, "Late Autumn".

Then it progressed to, "November".

Then it progressed to, "November 20th".

In a day where we really like to plan everything and have such busy lives, it's no wonder people want to know the specific day their baby will be born. Furthermore, it's important to have an accurate gestational age estimate because then we can better anticipate any special care the infant might need if born "too early" (I don't believe in babies being born too early or too late. I believe babies are born exactly when they're supposed to be, but some will need more healthcare support than others. All of life's experiences happen for a reason and I believe they are to be embraced even when they're less than ideal).

Anyway, understanding the concept of due dates and how accurate due dates are requires an understanding of the female menstrual cycle. So let's get a little background.

Day 1 of your cycle is Day 1 of your period (often called LMP, last menstrual period). This is the date used for calculating "Estimated Due Dates". A calculation called "Naegels Rule" asserts that a Due Date can be calculated with the following formula:

Due Date = LMP + 9 months + 7 days

Here's the issue... This is based off a 28 day cycle where the woman ovulated on approximately day 14 or 15 of her cycle. Even in this perfect scenario, the due date is just an ESTIMATE. Some babies need more time, some need less. But the truth is, some studies estimate only about 13% of menstruating women actually have a 28 day cycle.... Some are longer, some are shorter. Some may have a 28 day cycle but not ovulate til day 18 (and have a shorter luteal phase) whereas another woman might have a 28 day cycle and ovulate day 12 (and have a longer luteal phase). So already you can see, even within a 28 day cycle we could have up to a week of variation of conception dates.

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So according to Naegle's Rule, if you ovulated day 14 and conceived, on the date of conception you are exactly 2 weeks pregnant. So 2 weeks later when you find out you're pregnant because you missed your period, you're actually "4 weeks pregnant".

Now imagine You have a 32 day cycle and ovulated and conceived cycle day 21... Then on the date of conception, according to Naegles rule, you're 3 weeks pregnant. But actually you're the same amount pregnant as the mom who got pregnant on cycle day 14 but was "2 weeks pregnant"

I know it's confusing and I hope you're still with me, because it's important to understand this. This is actually dangerous. This method of estimation could lead to an inaccurate due date that makes the mother comfortable with scheduling an induction at what she believes to be 38 weeks pregnant but is really 37 weeks pregnant. That week of difference poses a huge risk to the infant as many (though certainly not all) 37 weekers need medical support. If the induction is being done for a non-emergent reason and a baby is born gestationally more immature than anticipated, this would be a seriously concerning situation. This is not to get into an induction debate, but forcing a baby out before they naturally signal they're ready (whether that's at 36 weeks or 41 weeks) can have negative consequences for the baby.

A study in 2013 (1) showed that although in the Western World we consider the "40 week mark" from LMP to be the "Due Date", it would be more appropriate to estimate the due date at 40 weeks + 5 day. In this study, one mother gave birth at "45 weeks + 6 days" from her LMP, BUT 40 weeks + 6 days after her OVULATION.

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Although Naegle's rule is still applied frequently today, due dates are often "changed" once an early ultrasound is performed to estimate the due date. Early ultrasound measurements do tend to be more accurate indicators of gestational age, but as the pregnancy progresses the measurements become less accurate as there becomes more room for variability among individual fetuses.

So we know that our predictions of due dates based off of LMP can be inaccurate and potentially dangerous. We know that ultrasounds done between 8 - 16 weeks gestation can be more much more accurate predictors of gestational age... but do they accurately predict your baby's birth date?

The answer is no.

Some sources suggest around 5% of women give birth on their due date. The rest give birth before or after, most birthing sometime within 3 weeks before their EDD and 2 weeks after, as a mix of spontaneous labors and inductions. Other sources point out that the average first time mother will spontaneously go into labor around 41 weeks + 1 day, even with perfectly knowing conception dates, etc.

The point? Due dates are not deadlines. Too often we are focused on a due date as a deadline, and your individual baby may not be on your same timeline.

This is where the concept of the Due Month comes in. I believe many mothers would have less anxiety and stress in the last month of pregnancy if they didn't have a "deadline" looming over them. Even if you're 100% comfortable going to 42 weeks gestation, you might paint that date as your "deadline" in your mind which can lead to anxiety.

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Anxiety, as we know, inhibits labor progression.

What if healthy mothers were focused more on a birth month or birth season? How would that change emotions and attitudes toward the end of pregnancy? While an accurate due date may be important for the mother and provider to be aware of when it comes to making healthcare decisions, maybe we don't actually need to share due dates with the world.

What happens when your due date comes and goes? Your phone starts ringing and dinging. Everyone is "just checking on you" to "see how your feeling" and casually asking if you have any "signs" of labor yet... which you probably do because.... well soft signs of labor can start weeks before labor... which is a whole other conversation.

So when you announce your pregnancy, I urge you to consider sharing your "Due Month" with family and friends to ease some of the stress.

"Due" April 29th? Tell people May.

"Due" June 15, say late June.

Just like so many parents like to keep the name they picked a secret, there is no shame in keeping a due DATE out of your mind and only speaking of due MONTHS, because odds are you'll give birth sometime before or after your due date. And that's a-okay.


Jaimie Zaki is a military wife, mother, Doula, Motherhood Photographer and IBCLC (International Board Certified Lactation Consultant) who is eager to help women birth and breastfeed with confidence.

Jaimie has personally experienced cesarean birth, VBAC in the hospital with an epidural, and a HBAC (home birth after cesarean) with no interventions. Learn more about Jaimie and why she is so passionate about supporting women through the childbearing year and beyond. Sign up for an on-demand childbirth or breastfeeding class with Jaimie today! Looking for one to one support? Book a free 30 minute consultation with Jaimie to see how she can help you reach your birth experience and breastfeeding goals!

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Pregnancy blog answers new mom FAQ how would due months change pregnancy instead of due dates and are due dates even accurate mother wonders with shrugged shoulders

Mother holds phone looking confused trying to calculate her due date and asks how long will i be pregnant. Doula Pregnancy Blog answers new mom FAQs on accuracy of due dates


Keywords: Due Dates, Naegles Rule, How to determine due date, are due dates accurate, what is my due date, when will my baby be born, LMP, Last menstrual period, ovulation and conception, menstrual cycles


Cited Source

1 - Jukic, A. M., Baird, D. D., et al. (2013). Length of human pregnancy and contributors to its natural variation. Hum Reprod 28(10): 2848-2855.

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