Your due date is wrong!
Yep! Well, it might be. In fact, more than likely.
How do I know this? Because the EDD – Estimated Due Date – is one of the biggest [lies/disasters/ trick /hoaxes etc] to affect pregnant women today. And, even though people who should know better (doctors, midwives etc), know better, this lie still stands. It’s a bit like the whole Santa thing, but far more damaging.
WHAT? What? Tell me!!!
And because of this, you’re setting yourself up for a ton of stress in the last month or so of your pregnancy.
So why is it wrong?
The most commonly used method to calculate due dates is also the most inaccurate. Seriously. Let me explain.
The due date system that is widely used by doctors and midwives is based on Naegele’s Rule (Franz Karl Naegele OB, 1778–1851).
Naegele’s Rule
According to Naegele’s Rule, the standard definition for gestational term is 266 days from conception to the date of the baby’s birth. This is also defined as 280 days, or 40 weeks, from the first day of the mother’s last menstrual period, a definition which assumes that the mother ovulates on day 14 of a 28 day menstrual cycle. The actual formula used to calculate estimated due date (EDD) is:
(LMP + 7 days) – 3 months = Due Date
This is the formula that was probably used to give you your due date.
In fact, Naegele’s theory originated from a guy called Harmanni Boerhaave, a botanist who in 1744 came up with a method of calculating the Estimated Due Date (EDD) based upon evidence in the Bible that human gestation lasts approximately 10 lunar months.
I don’t know about you, but for me, there is so much wrong with that last sentence, that when I first read it my jaw bruised my big toe. Considering what’s at stake, this calculation method doesn’t exactly inspire me with confidence… and here’s why:
The Bible?
Like HOW long ago?! You would think that given how many people have been born since then, that we would have built up a pretty good picture of this whole human gestation thing. You know maybe, even taken the time to take a closer look to see if the assumptions we’re using are appropriate…. You’d think?!
“…based on evidence in the Bible…”
Since when is the Bible a reliable source of evidence? Or indeed a reliable authority on the passage of time. The Bible said that the Earth was created in seven days, and yet most of us have accepted that that is highly unlikely, if not downright impossible. But yet here we are using an idea from the Bible to tell women when they can expect to bring their mini human into the world. What makes this unforgivable in my mind is that this rule is NOT based on scientific or empirical evidence or research. Yes, you read that right… it is NOT based on empirical evidence or research. None whatsoever. Not even 100 year old scientific research when hospitals were a lot muckier than they are now. No! We’re talking about times when women gave birth along side cattle and donkeys in barns by candlelight.
Can you tell that this makes me mad? But, it gets worse…
“…that human gestation lasts approximately 10 lunar months”
Lunar cycles?!! Seriously? OK.. Well let’s follow this line of thought and see where it takes us.
A quick check online will tell you that there are five different lunar months ranging in length from 27.3 days to 29.5 days, but the one that is widely used is the synodic month, which wikipedia describes as “how long it takes on average to pass through each phase (new, half, full moon) and back again” and which lasts 29.5 days. So if the Bible assumes that human gestation lasts approximately 10 lunar months then according to my calculations 10 x 29.5 = 295 days.
Now if you recall, Naegele’s Rule says that human gestation is 280 days. So, given that 10 lunar months gives us 295 days… we’re already hitting some problems… Let’s do some more maths..
295 – 280 = 15 days difference
This is 15 days LONGER than the 280 days gestation period that is being used and that we’ve been lead to believe is average. That’s 2 weeks! So, even the rule is confused!
Already we can see that using Naegele’s Rule, gives us 2 week slacks when it comes to the due date calculation.
But hang on, I’ve not finished…
28-day menstrual cycle
Naegele’s Rule is based on the idea that our menstrual cycle lasts 28 days. Hands up – whose cycle DOES NOT last 28 days? Most of room, huh? Thought so… We all know that our menstrual cycles vary in length… I know mine is nearer 34 days than 28 days. Do you know what yours is? It’s accepted that menstrual cycles can last anything from 22 to 35 days. So if your cycle is NOT 28 days then you need to tweak the formula. If your cycle is 34 days, then you would need to add 6 days to your due date (34 – 28 = 6), whereas if your cycle is 26 days, you would take 2 days off (28 – 26 = 2).
Ovulation
And finally, Naegele’s Rule assumes that we ovulate exactly half way through our 28-day cycle … 14 days in. But ovulation is not always half way through a cycle as it can be affected by things like whether you’ve recently come off the pill, or if you’re stressed, ill or experiencing a disruption of routine. In other words; LIFE!
Now that we’ve peeked behind the curtain that is Naegele’s rule…. What is your level of confidence in the due date calcualtion that you’ve been given as your Estimated Due Date?
Isn’t it time to find a date that is more realistic? And perhaps one that is based on a method that is backed by science?
The 3 alternative due date calculation methods
During the podcast I said that I’d share with you 3 other due date calculation methods that are based on scientific research carried out in the twentieth century. But before you can work out a more realistic due date using these methods, you’ll need to be armed with certain pieces of information. You will need;
1. The start date of your last menstrual period (LMP)
2. Your average cycle length (in days)
3. What number pregnancy you’re on… a pregnancy being defined as ‘A woman who has given birth to an infant, liveborn or not, weighing 500g or more, or having an estimated length of gestation of at least 20 weeks.’
The 3 alternative due date calculation methods are based on the work of
- Park (1968)
- Nichols (1985a)
- Mittendorf et al (1990)
Now during the podcast I mentioned that I would share with you a cheat sheet with all of the formulas from each of these due date calculation methods. The cheat sheet contains the formulas for each of the alternative due date calculation methods so that all you need to do is to work it out.
To get hold of that cheat sheet simply click on the button below. When you sign up below you will also receive emails to help you to feel calm and confident as you approach your birth.
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Resources & Further Information
During the podcast I mentioned that I would share with you my sources of information and research, well here they are..
Postdates: Separating fact from fiction
My favourite article as it’s nice and detailed but clear and easy to read:
Calculating Due Dates and the Impact of Mistaken Estimates of Gestational Age
A timely birth – Midwifery Today
This is a fantastic and very thorough article on due dates, inductions, risks of waiting etc and includes all the latest scientific, evidence-based research.
Did you work out your alternative due dates? How different are they? Has knowing this changed how you might plan the last month of your pregnancy, like when to stop work? What have you decided to do in terms of telling friends and family about your due date? Let me know in the comments below!
Alexia supports families planning pregnancy and birth. She helps them to overcome their fears and feel calm and confident about birth and pregnancy.
Alexia also trains birth professionals in the Fearless Birthing, a unique approach to birth preparation that is ideal for those who have fears around birth.
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I just listened to your podcast. I appreciate it! I have to say though, I respect the fact that other people have different beliefs but I feel like the way you spoke of the Bible was a little off putting. I say this from a place of kindness. I’m REALLY surprised in this day and age of respecting religions that Christian beliefs were criticized.
I totally believe that due dates are SO wrong and I’m going to stand up for myself better this time around (my 5th). Going to check out the due date cheat sheet. Thank you!
HI Lizzie,
Thank you for taking the time to drop by. I’m sorry you feel like about what I said. PLease be assured I was not criticising Christian beliefs so I’m sorry that’s how it sounded. Thank you for letting me know your thoughts with kindness – much appreciated. I’m pleased that you found the due date podcast useful and I hope that having a better understanding of your dates will hellp you to stand up for yourself. Good luck!
Alexia
I was raised in a strict christian family and it’s a shame you took it that way but honestly, to people who don’t consider the bible a factual text (and even Christians should be well aware it isn’t), it is extremely offensive and dangerous for medical professionals to use it as one. That is much more offensive than any religious slur.
Hi Elizabeth
Thanks for taking the time to commment. I find it diffificult to comprehend in today’s scientifically driven world that something as important as our very own gestation length and therefore expected arrival date isn’t widely supported by a research-based approach. Women and children’s lives are at stake! This is what I find shocking.
Hi Alexia,
I’ve listened to your podcast on due dates and just wondered what your thoughts were on the fact that the calculated due date is usually adjusted after the first scan based on the size of the baby? For example my calculated due date based on dates was 18th November but after the scan it was changed to 14th november. Surely they must have reason to believe the measurements are more accurate than any calculations?
Many thanks
Emma
Hello Emma,
You might want to read the link I’ve included in the blog poast called “Calculating Due Dates and the Impact of Mistaken Estimates of Gestational Age” as it discusses this very point as well as loads more about due dates. But essentially, the ultra-sound measurements become more inaccurate as your pregnancy progresses – the first one is the most ‘accurate’ at being +/- 3-5 days. Also, the measurements that are taken aren’t always taken accurately, which means they can be interpreted with a margin of error. One lady shared with me that her midwife admitted that as the baby was moving when she was taking the measurements, if she’d written down one of the other measurements, it would have changed her due date by as much as 6 days.
I think the thing to remember is that baby will arrive when baby wants to and that it’s probably better to think of a due month than a due date… and to be prepared for baby not being ready to make an appearance until 42 weeks… because that would be OK! as long as all your health metrics are ok of course.
I hope that helps!
Alexia
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Hi Alexia,
I’ve just discovered your podcast and am really enjoying listening to it every day. I find i’m getting way more excited about what’s ahead which is a pleasant surprise. Regarding due dates, My cycle was always in time with the moon. I always bled with the full moon until we started talking about TTC and it swapped to the new moon in the space of 2 months. I’ve been tracking my basal body temp and i’m fairly certain i ovulated with the full moon. Therefore I have a sneaking suspicion i will be due 9 moons from there. I’ll let you know what happens, i’m only 1 month in. I think the moon idea is much older than the bible. When there was far less light pollution from cities i think its possible more women were in sync with the moon.
I wanted to add i agree with you about the abhorrent idea of basing this on something in the bible. You would be hard pressed to find too many books that oppresses women more.
Hi Johanna,
Thanks so much for dropping by to let me know about your cycle. That’s really interesting how it’s in sync with the moon, and then how it changed! Wow! I’d love to know how things pan out for you… It will be really interesting to see, espacially given how in tune with the moon you are. Have you calculated the date that is 9 moons away? And how does this commpare to your due date?
I’m sure you’re right, that before mankind started losing its connection to nature, that we were more in sync with the moon. I wonder what else we might be missing out on that we simply do not know about.
I’m really pleased you’re enjoying the podcast – keep me posted on how things go for you
Alexia x
Although calculating the EDD using Naegel’s rule can be inaccurate due to variations in menstrual cycle duration and time of ovulation, the EDD is re-assessed during the dating scan using a crown-rump length that is a more accurate assessment of the EDD based on the calculated gestational age from the dating scan. If there is a significant difference, the EDD is adjusted accordingly.
Hi Stephanie,
Yes, you’re right. However, dating scans become more inaccurate as the pregnancy progresses with the dating scans taken after 30 weeks having the potential to be up to 3 weeks off. The most accurate dating scans are those taken undwer 10 weeks. Also, the position/movement of the baby when the scan measurements are taken can also affect the date. I’ve one lady tell me that a nurse admitted that she could give her a due date that differs by as much as 5 days depending on which measurement she took, and she asked her which due date she wanted! I think the thing we need to bear in mind is that baby’s aren’t considered ‘ripe’ at a certain point in time because we all grow at different rates. We should simply let go of this obsession over a particular date and just allow babies to arrive when they’re ready.
Hello – I can’t seem to access the PDF with ways to calculate due date from this page. An error message comes up. Is there anywhere else that this is available? Many thanks.
Hello Vicky. I’m sorry you couldn’t get hold of it. The link should be fixed now.
Alexia
Ah delighted to have found your site! Wish I’d found it before to use as backup although, fortunately, I’m a serial googler when anyone tries to tell me what I ‘have’ to do with my body, so was saved from interference last time.
First pregnancy spontaneous labour started 5 days after EDD and despite hypnobirthing and birthplan, the one thing we hadn’t researched thoroughly enough was my waters being broken. What a nightmare! Should have stuck to our guns on no interference but presumed that would be harmless. I’d been at 7cm for a few hours so it was suggested because I hadn’t progressed, despite being calm myself and baby also. Said it would be another 5 hours before arrival so had pethadine and waters were broken, I stood up, baby dropped like a stone and became distressed. Within an hour, one very painful episiotomy later and a baby needing pethadine antidote later, all was calm again, but what an unnecessary faff.
Second time I was prepped for that! Wasn’t prepped for the “right we need to book in your induction” two weeks before EDD for 1 week ‘late’. Umm hang on, I’m not having one, isn’t it my choice? Well, yes, but it’s very dangerous.
Insisted it was not to be booked and I’d cross that bridge when I got there. Referred the midwife to my rights of induction OR monitoring and informed her of the risks associated with induction. Never saw her again. Anyway, the nagging continued, every day beyond EDD family were getting impatient making me feel pressured into induction. Held firm. Baby rocked up within 90 minutes of spontaneous delivery, cool as a cucumber and happy as anything, a casual 11 days ‘late’.
Based on a 10 cycles calculator she was bang on.
If I’m lucky enough to be blessed with another (we’d like four), I’ll most definitely give a much more vague idea when people ask!