The Problem With Estimated Due Dates


There is a problem with estimated due dates and I think you need to know about it!

Considering it is the single piece of information that dictates your baby's eviction from the womb, it is actually pretty important to understand how your provider arrives at the date.


We’ve all been raised to believe that a pregnancy lasts for 40 weeks. And like a lot of information that comes out of a medical system built upon the foundations of patriarchy and male dominance, we are expected to just take it as FACT!


But the truth is the estimated due date (EDD) we are given at our first prenatal is often very wrong.


That date comes and goes and we’re still pregnant. Not a niggle or a sign of labour insight. We have been fighting off offers of stretch and sweeps and membrane ruptures and other interventions for the last few weeks and now our OB is getting serious about inductions and booking us in for a C-section because we’re suddenly overdue or postdates.


It seems that our baby is about to be evicted from our body whether they are ready or not…based on a date that is often very wrong!


The 40-week due date that the medical system has adopted and now treats as the gospel is based on ‘Naegele’s Rule’, a theory that was put forward in 1744 by a botanist (yep, a botanist) named Hermann Boerhaave.


Now to understand Naegele’s Rule we need to delve into a little maths, a little science…and the bible...


Please bear with me while I attempt to explain the background of this ‘rule’ as I am neither mathematically-minded nor religious.


Old mate, Hermann, based his theory on ‘evidence’ from the Bible (yep, the Bible) that human gestation lasts approximately 10 lunar months. This gem comes from the New Testament account of the birth of Christ and his gestation from the Feast of the Annunciation in March until Christmas Day.


This theory and the formula that is still used today was adopted and published around 1812 by German Obstetrician Franz Naegele and has been the accepted method of calculating due dates ever since.


Now according to Naegele’s Rule gestational term is defined as 266 days from conception to the date of the baby’s birth or 280 days (or 40 weeks) from the first day of the last menstrual period (LMP).


For this formula to be even close to correct, it assumes that all people ovulate on day 14 of a 28-day menstrual cycle.


I don’t know about you, but my cycle is hardly clockwork so immediately this formula does it work for me.


Let’s just back up a minute and revisit the fact that the formula was based on lunar months (I’m not even going to touch on the fact that the evidence behind this comes from the great scientific research of the Bible).


There are actually five different lunar months ranging from 27.3 days to 29.5 days in length, however, the synodic month (from New Moon to New Moon) is the one that is widely used as a measure of a standardised lunar month and this is 29.53 days long. If we use this month as the basis for the calculation this makes the Bible and Naegele Rule’s 10 lunar months approximately 295 days.


Now, I know it feels like we are wading through the weeds with this but for those who are managing to follow along, 295 days is a full 15 days longer than the 280-day gestation we’re all led to believe is the norm. That is 2 weeks, 2 whole weeks, longer than we have been told is normal for human pregnancy. Two whole weeks longer for your baby to fully develop before it gets its eviction notice!


Just to confuse things a bit more let’s back up to the 28-day cycle that this formula is based on. It’s widely accepted that menstrual cycles can last anywhere from 22 days to 35 days and still be considered ‘normal’. We also know that most of us do not release an egg at exactly halfway through our cycle (day 14). In any case, ovulation can be affected (delayed or hurried along) each month by hormones, stress, illness or even disruption of routine.


So…if your cycle is anything other than 28 days with ovulation on day 14, Naegele’s Rule will give you an inaccurate EDD.


This means that if you are aware of your cycle and know the day you generally ovulate, your health care provider should adjust Naegele’s Rule to assure your EDD is more accurate

Needless to say, this very rarely happens!



Let’s head back into the weeds for a minute and revisit our friend, The botanist who came up with the original calculation. Boerhaave stated in his lecture ‘On Conception’ in 1744,

“that for of 100 births altogether 99 came about in the 9th month after the last menstruation. By counting one week after the last period and by reckoning them 9 months of gestation from that time”.

From this statement, it is unclear whether he was referring to one week after the start of the LMP or after the end of the LMP. It was Naegele who suggested that the formula should refer to the first day of the last LMP as it was more,

“plausible as it is mid-cycle and therefore likely to coincide with ovulation”.

So we really have no firm decision one way or another, just a little more confusion to add to the mix.


There are a few other formulae available for calculating EDD, however, these are very rarely utilised.


The 3 alternatives are based on the work of Park (1968); Nichols (1985); and Mittendorf et al (1990) Nichols and Mittendorf offer different formulae based on whether the calculation is for your first birth or subsequent births and all four formula basically place gestation at between 280-290 days.


The calculation of the expected due date and the gestational age of the baby is not just important for the planning and anticipation of the expecting parents but has considerable medical implications associated with induction of labour. It is common obstetric practice (just want to raise the patriarchy flag once again) to induce labour within 7–14 days past the expected due date, usually with the reason of avoiding the very rare risk of fetal death, due to failing placental function.

Due dates and gestational age based on dates assumes an invariable 28‐day cycle and that ovulation always occurs mid‐cycle. While this is a reasonable working assumption there are many variations between and within birthing people that are very rarely taken into account.



In my opinion, all of the formulae are flawed as they are trying to pin down something that has so many variables attached to it that it is almost impossible to standardise. For example, none of the formulae takes into account that gestation length is also affected by other factors including ethnicity.

Knowing that only 3-5% of babies are born on their due date, I always advise my clients to treat their EDD as the day that they use to calculate their ‘due month’.


Term (when a baby is considered ready to be born) is considered to be anywhere between 37 weeks and 41 weeks and 6 days. This means there is almost a 5-week window when the baby may arrive. Usually, you will find your EDD is somewhere in the middle of this 5-week window. Knowing this and understanding the implications of it can make it a lot easier for pregnant people close to their EDD to fend off their providers scare tactics of recommending induction based on an arbitrary date. It can allow them to feel confident in allowing their body to tell them when it is ready to birth their baby.


So how do you feel about your EDD now? Are you confident in its accuracy?


Does this information give you the confidence to tell your doctor that Naegele’s Rule is often very incorrect so you won’t be using it to predict your baby’s EDD?


Jenine Ellis is a true gem! Someone, this planet is so lucky to have, why? Because she demonstrates so beautifully what a life of service truly looks like. She is dedicated to Birth and all Birthing People, a Birth & Postpartum Doula, Jenine is also Hypnobirthing Practitioner, Placenta Encapsulator and Womb Wellness Practitioner and she has an online market place full of beautiful and helpful birth and postpartum tools. She is a passionate and seasoned Birth Worker who has been providing care for her community for 15+ years. Servicing the Melbourne and Surrounding Region, you can find out more by visiting www.graceandivy.com.au.


Jenine is one of our beautiful Birth Workers and Stockists, learn more about her by visiting the Directory and her profile HERE.

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