You're pregnant. Wow! Congrats! I'm celebrating you and your growing family right now.

I imagine one of the first things you’re wondering is… How do I calculate when I’ll actually be giving birth and meeting this tiny new human? 

Next Steps: Understanding Your Menstrual Cycle and Due Date

Perhaps you’ve gone on Google, found Naegele’s Rule and used that formula to calculate what you believe to be your Estimated Date of Birth (EDB). 

Or maybe you’ve gotten an ultrasound and the doctor has ‘determined’ your Gestational Age based upon that.

Just Because Everyone Does It, Does't Make It Right!

Sorry to be the one to break it to you, but calculating your due date in these ways isn't necessarily accurate. And potentially even harmful! Not to worry, though — you’ve come to the right place for truth! I'll lay it all out for you, so that you can be prepared and make informed decisions for your growing family.

In this blog post, I’m going to show you what's the normal mainstream process, and the gaps and pitfalls in going along with it. Then in the next article, I'll show you a much more accurate, conscientious and healthy way of determining your estimated date of birth. I’ll even share a whole bunch of paradigm-shifting perspectives surrounding the estimated ‘due dates’ and how you can navigate this part of pregnancy with mindfulness and ease. 

And guess what? I have even put this information together into a comprehensive video course called 'Discover the Truth About Your Due Date' - and it's FREE! Prefer to watch personal videos rather than reading through all the details? Sign up for the free video course here.

The Mainstream Way Most People Calculate Their Due Date

Let's break down Naegele's Rule, the standard method widely used among hospitals, medical care providers, birth centers, midwives, doulas and found all over the Internet for estimating a baby's due date. It is based on the mother's last menstrual period (LMP), also known as the last normal menstrual period (LNMP), rather than her actual date of conception or ovulation. 

Here's how Naegele's Rule works

1. Begin by identifying the first day of the last menstrual period. 
2. Subtract three months from the LMP date. This accounts for the typical length of a pregnancy, which is about nine months.
3. After subtracting three months, add one week to the result. This is meant to account for variations in cycle length (but doesn’t fully do so). It aligns with what is considered a typical 40-week pregnancy duration. 
4. Finally, adjust the calculated date for the correct year. 

So for instance, if the first day of your last period was June 15th, 2024, then according to Naegele’s Rule, we would assume your estimated date of birth to be March 22, 2025.

Here’s the Naegele's Rule formula breakdown:

• First day of your last menstrual cycle → June 15, 2024
• Subtract 3 months → March 15, 2024
• Add 1 week (15th + 7 days) → March 22, 2024
• Adjust for the correct year → March 22, 2025

Now that you understand how most people calculate EDB, continue on to the next lesson to realize why this doesn’t actually work.

Dispelling the Myth

Let's debunk a common myth right from the start: this traditional method of calculating your due date is flawed. Relying solely on outdated formulas based on the first day of your last menstrual period (LMP) fails to account for the unique rhythms of your body and conception cycle.

It's essential to recognize that Naegele's Rule assumes a standard 28-day menstrual cycle, and also assumes a woman becomes pregnant on the day of ovulation, which typically occurs around the midpoint of the menstrual cycle.

In a typical 28-day menstrual cycle, ovulation usually occurs around day 14. Therefore, Naegele's Rule calculates the estimated due date (EDD) based on this assumption, counting forward 280 days (or 40 weeks) from the first day of a pregnant woman's last menstrual period (LMP). Obviously, all of this definitely does NOT apply to every woman. 

Not Every Woman Ovulates At the Same Time

For some women, ovulation may occur earlier in the menstrual cycle than the typical midpoint. For example, women with shorter menstrual cycles (24-26 days) may ovulate around day 10-12 of their cycle. In such cases, conception could occur shortly after ovulation.

Conversely, some women may experience delayed ovulation, particularly those with longer menstrual cycles (35-40 days). In these cases, ovulation may occur later in the cycle, around day 20-25. If conception occurs shortly after ovulation, implantation may occur later than the typical 6-10 days post-ovulation.

Here are some specific examples I received from one of my midwifery mentors, Whapio, which illustrate very clearly the discrepancies we often see when using Naegele’s Rule:

• Example A) 31-Day Menstrual Cycle: Suppose a woman has a 31-day menstrual cycle. Using Naegele's Rule, her estimated due date would be three days earlier than expected by nature. This discrepancy arises because the formula assumes ovulation occurs on day 14 of a 28-day cycle. With a 31-day cycle, ovulation typically occurs on day 17, resulting in an incorrect estimated due date.
• Example B) 32-Day Menstrual Cycle: For a woman with a 32-day cycle, ovulation would occur around day 18, not day 14 as assumed by Naegele's Rule. This difference of four days can impact the accuracy of the estimated due date and may lead to unnecessary inductions or interventions if not properly accounted for.
• Example C) 35-Day Menstrual Cycle: Consider a woman with a 35-day menstrual cycle. Her ovulation would occur later in her cycle, around day 21, resulting in an estimated due date that is off by about a week according to Naegele's Rule. Imagine inducing a baby a week early for no good reason and robbing them of vital gestation inside their mother’s womb. 

Outliers Are More Common Than You Think

All of these scenarios prove the inaccuracy of using this generalized formula and applying it to women with differing menstrual cycles. Now let’s dig deeper and explore a few other ways utilizing Naegele’s Rule, the way almost every single due date today is calculated, can and often does totally throw things off. 

• Inconsistent Periods

What if you don’t experience consistent cycles? Maybe some weeks your cycle is 28 days, other times it’s 30 days, and still other times you have a 33 day cycle. How do we utilize a formula to determine your due date?

• Can’t Remember the Date of Your Last Period

What if you realize you’re pregnant, but perhaps it wasn’t planned and you weren’t actively tracking your menstrual cycle? Do you guess and give a rough date for your LNMP and then base your entire pregnancy’s developmental markers upon something that’s very likely incorrect?

• Generalized Assumption of the Date of Conception as During Ovulation

Naegele's Rule ultimately is making a big assumption by generalizing women to conceive and become pregnant on the day of ovulation, which typically occurs around the midpoint of the menstrual cycle.

In a typical 28-day menstrual cycle, ovulation usually occurs around day 14. Therefore, Naegele's Rule calculates the estimated due date (EDD) based on this assumption, counting forward 280 days (or 40 weeks) from the first day of the woman's last menstrual period (LMP).

But did you know that the period of ovulation in a woman's menstrual cycle is NOT the only time that a woman can become pregnant, despite common misconception?

Continue on to the next blog to learn about women’s menstrual cycles and all the important details on ovulation.