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Is Your Due Date Wrong? Here’s How to Make it More Accurate

We often talk about due dates like they are dependable facts…but are they? Short answer: no. In this blog post, learn why due dates are just an estimate, how to make them more accurate, and what you can do to reduce your anxiety surrounding going past your due date.

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If there’s one thing people focus on during pregnancy, it’s due dates. A woman’s due date tells people (roughly) when the baby is coming, how far along she is, and can even determine if and when a woman decides to get induced.

But what if due dates weren’t as reliable as we thought they were?

Well, that’s reality. Due dates are not very accurate, at least not the way we usually calculate them.

Why Due Dates Aren’t Very Accurate

So why aren’t due dates accurate? Largely, because of math. Don’t be scared away. It’s pretty simple, really.

Most care providers, whether they be midwives or OBGYNs, use what’s called Naegele’s Rule. (That’s pronounced NAY-guh-luh, by the way.) 

Naegele’s Rule is where we get the idea that pregnancy is 40 weeks long. 

Using Naegele’s Rule

To calculate your due date using Naegele’s Rule, you need to know the first day of your last menstrual period (shortened to LMP).

Then it goes like this:

LMP + 7 days + 9 months = estimated due date (EDD)

Alternatively you can subtract 3 months if that’s easier:

LMP + 7 days – 3 months = EDD

Since months are not all the same length, though, it’s more consistent to just add 280 days:

LMP + 280 days = EDD

So if the first day of your last menstrual period was January 29, you’d add 280 days and your estimated due date would be November 4. 

Problems with Naegele’s Rule

Naegele’s Rule came from the studies of two professors: Hermann Boerhaave and Carl Naegele. They both suggested the math that I explained above…with one exception.

Neither one specified on paper (so we have no way of knowing) if the LMP date they used was the first day of the last menstrual period, the last day, or some other day of the cycle! That creates a pretty big problem.

The average woman’s menstrual cycle is 28 days. So using an LMP of the first day of a cycle would give you a due date an entire month earlier than using an LMP of the last day of a cycle.

You see the issue?

In addition to not knowing which LMP to use, Naegele’s Rule assumes all women have an average cycle length. But many do not. Some have much longer cycles, some much shorter.

Women also ovulate at different points in their cycle. The average is day 14 but that’s not always the case.

And finally, even after an egg is fertilized, one woman’s egg may take longer to implant in her uterus than another woman’s, which also would change the real due date.

Alternate Ways to Determine Due Date

While Naegele’s Rule is the most common way to determine due date, it isn’t the only one. And others are usually more accurate.

There are two main alternatives to Naegele’s rule.

Early Ultrasound

The first is to use an early ultrasound. If an ultrasound is done before 20 weeks (ideally between weeks 11 and 14, according to one group of researchers), the measurements are usually a more accurate predictor of due date than a calculation based on your LMP.

Nichol’s Rule

The second option is to use Nichol’s Rule. Carol Wood Nichols was a midwife and professor who came up with a new way for calculating due dates. Her rule assumes that the average first-time mom will give birth after 41 weeks and 2 days, not 40 weeks. (That’s what happened with my first!)

For first-time moms, Nichol’s Rule goes like this:

LMP (first day) + 12 months – 2 months – 14 days = EDD

Again, we run into the problem of months being different lengths. Depending on where you’re at in the year, it might be most accurate to use 30 days or 31 or days in a month for your calculation.

I was due in April with my first daughter so the Nichol’s calculation would have looked like this for me:

LMP + 365 days – 62 days – 14 days = EDD

So my “actual” due date according to my care providers was April 1, but my Nichol’s due date – and the day my baby actually came – was April 10.

Nichol’s rule is slightly different for moms who have given birth before. It looks like this:

LMP + 12 months – 2 months – 18 days = EDD

That assumes that second- or third-time moms (or more) will often have their baby at 40 weeks and 5 days.

Adjusting Your Nichol’s Due Date for Cycle Length

She also included a way for women with cycles different than 28 days to adjust their EDD. To make the adjustment, find the difference between your cycle length and 28 days.

Then if your cycle is longer than 28 days, add that difference to your original Nichol’s EDD. If your cycle is shorter than 28 days, subtract the difference from your original Nichol’s EDD.

That looks like this:

Cycle longer than 28 days: EDD + (# of days in cycle – 28) = new EDD

or

Cycle shorter than 28 days: EDD – (28 – # of days in cycle) = new EDD

So what’s your Nichol’s due date? Let us know in the comments how different it is from the due date your care provider gave you!

Let’s Talk About Babies Being “Early” and “Late”

It’s ingrained in us to talk about babies’ births as “early” and “late” depending on when they were born in relation to their estimated due date. I do it too, even though I know the truth about due dates.

So real quick, let’s talk about what “early” and “late” really are.

According to current definition, this is how things are broken down:

  • A baby born before 37 weeks is considered “preterm” or “premature.”
  • A baby born between 37 weeks 0 days and 38 weeks 6 days is considered “early term.”
  • A baby born between 39 weeks 0 days and 40 weeks 6 days is considered “full term.”
  • A baby born between 41 weeks 0 days and 41 weeks 6 days is considered “late term.”
  • A baby born at 42 weeks and 0 days or later is considered “post term.”

These time frames are widely accepted and are useful in determining how to care for a pregnant mom and her baby. That said, it all depends on how accurate the due date was in the first place.

If a due date was off by even 10 days, that could be the difference between a full term pregnancy a care provider would likely not worry about and a pregnancy labeled as “post term,” which raises intense concerns in many care providers.

It could also mean that a doctor encourages a woman to get induced since, according to her estimated due date, she is “full term” but in actuality her baby has barely reached early term and may not be ready for life outside the womb yet.

This is why induction can be such a problem.

It’s also why discussions about being late and worries of post term pregnancies are often unfounded (or at least misrepresented).

How to Avoid Anxiety About Your “Due Date”

As I mentioned, one problem with due dates is the possibility of misjudging a baby to be more developed than they really are. But the other big problem with perceiving due dates as reliable and concrete is what it can do to a mother’s mental health.

If you expect your baby to be born on June 3, you might be disappointed on the morning of June 4. And by June 12 you might be stressed about the safety of your baby.

But you don’t have to be!

Chances are, your baby’s most likely birth day was actually June 12 all along! And even if it wasn’t that day exactly, your baby is almost certainly safe and healthy even after you cross that 40-week line.

So we need to change how we think about due dates – for ourselves and those around us.

Three Ideas

Here are some ideas for how you can reduce your anxiety about your due date:

1. Rather than using the due date your care provider gives you (which will be according to Naegele’s Rule), calculate your own! Use the Nichol’s rule to get a better idea of when your baby will be born.

2. Talk about your due date differently. In your own mind, think about your “ESTIMATED” due date (not just “due date”) to remind yourself that it’s just that – an estimate. To take it a step further, you can tell yourself – and others – that your due “date” is actually a range of time, like “beginning of April” or “mid-November.”

If it helps you, you can even use different words entirely. Instead of “due date,” try “birth month” or “birth time.”

3. Don’t get a third trimester ultrasound unless there is a medical reason for doing so. While early ultrasounds can predict due dates well, late ultrasounds cannot. A third trimester ultrasound is more likely to cause stress than be helpful.

Babies grow at different rates and to different sizes, and that’s normal. Plus, measurements by ultrasound, especially late in pregnancy, are fairly inaccurate.

Conclusion: Don’t Depend on Your Due Date

In summary, estimated due dates are not consistently reliable. The typical way of estimating, Naegele’s Rule, assumes a 40-week pregnancy (not actually the average length) and that all women have a 28-day menstrual cycle (not true).

Instead of Naegele’s Rule, you can get an early ultrasound to determine your due date or use Nichol’s Rule, which assumes that first-time moms will go 41 weeks and 2 days and second-time moms (and beyond) will go 40 weeks and 5 days.

Finally, knowing what we know about due dates, we can talk about them differently. We can stop using “early” and “late” as often (and arbitrarily) as we do. And we can refer to “birth months” or estimated due dates as being “beginning of July,” for example, rather than a specific day.

With all of that in mind, we all can stress less and enjoy pregnancy more.

Until next time,

Allison

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