Calculations of Confusion?!*!?


So when I first went to my Medical Doctor I asked her why we calculate pregnancy based on a woman’s last menstrual period??!!  She just smiled and said “Yeah I know it doesn’t make sense, but that’s how it’s done?!”

Being the curious type I decided to find out the reasons behind this and if there was a more accurate method of calculating.

The answer I figured was — generally women know the dates of their periods and are less certain of their ovulation… so using the LMP was a best guess formula.  And of course having a basic idea is better then no idea!  Especially when there are perceived risks going post-due as the placenta will start functioning less optimally after 42 weeks.

With some digging I found out that in the 1800’s a German obstetrician called Franz Karl Naegele (1778–1851) fixed the length of pregnancy at ten lunar months – nine calender months – or 280 days… and now all pregnancies are dated according to this “Neagele’s Rule”… a simple calculation of LMP – 3 months + 7 days… and that’s why we do it.

Now if all women had a precise 28 day cycle this would work to some extent… In my case I do have a fairly bang on 28 day cycle and have for many years…  But as you can see from the above link – this method makes a couple other assumptions around lengths of calendar months and gestation… after all only about 5 % of babies are born on their estimated birth date.

Ultrasound dating is considered the most accurate method of dating… however also has it’s own margin of error … and depends greatly on the skill of the technician, size of the patient, position of the baby & timing.  Through my readings I found out that an ultrasound performed in the first trimester between 10 and 13 weeks, can indicate fetal age within a range of 3 – 5 days. At around 20 weeks, the margin of error is 7 – 10 days. By the third trimester, an ultrasound date can be off by as much as 3 weeks in either direction. Therefore, a baby that appears “term” (40 weeks) may be anywhere from 37 – 43 weeks… WOAH!

We will be getting our first ultrasound – 18-20 week – the anatomy scan –  in a few weeks and we are so excited to get our first glimpse of our little bean!

Other Dating Methods:

Dr. Mittendorf’s Formula – is based on his observation that most first time mothers gave birth 8 days later than the EDD calculated by Naegele’s Rule.  Women who had given birth before (multiparous) on average gave birth 3 days past their Naegele EDD.  African American women tended to give birth 8.5 days earlier than Caucasian women.

The Mittendorf formula is: (LMP – 3 months) + 15* Days = EDD

* Add 10, rather than 15, if mother is non-Caucasian, or multiparous

Parika’s Formula challenges Neagele’s Rule and claims to calculate more accurately — for those that have a longer or shorter then 28 day cycle — using the following formula:

EDD = LMP + 9 months + (Duration of Previous Cycles – 21 days)

More info on the web that digs deeper into these dating issues:

Issues in Pregnancy Dating: Revisiting the Evidence

The Lie of the EDD: Why Your Due Date Isn’t when You Think

How Accurate is Your Due Date?

Naegele’s rule: a reappraisal

The conclusion I have come to is with all the variables involved, there is no one definitive way to tell when a baby is “due”.  Some babies need to “bake” longer, and some are perfectly happy birthing into this world sooner.  About 80% of expectant women have full term births (37-42 weeks maturity). Only 20% give birth outside the regular parameters, and most are early.

As my doctor said – “The truth is babies come when there ready, we just need to know approximately when.”

Which then begs the question as to why induction is so widely used between 40-42 weeks… but that’s a whole other blog entry!!

What’s stuck out to me as most important during my research is little is known about post maturity (beyond 42 weeks), and although it is very rare to go past a few days (42 weeks +3-5 days), once past, due to the known risks rigorous monitoring is necessary to ensure the placenta is functioning well and baby is not in distress — including heart activity & ultrasound (checking amniotic fluid volume and babies movement), and intervention is more likely and necessary if spontaneous labour doesn’t start up on it’s own.

I especially like what Michel Odent – retired MD said –  “According to traditional wisdom in rural France, a baby in the womb should be compared to fruit on the tree. Not all the fruit on the same tree is ripe at the same time…we must accept that some babies need a much longer time than others before they are ready to be born.”


The next confusion was determining the beginning and end to trimesters.

Obviously 9 months divided by 3 is 3!  But depending on what book you read or website you browse it’s a bit different??!!

And really 9 months is a bit of a misrepresentation!  280 days divided by 30 (average month) is 9.333333… and if you think of a woman’s cycle – it’s measured by 28 day periods – which are lunar months – and there are 10 of them!!

I did find that trimesters can be calculated in three ways (which is likely the reason for the confusion).

I found this website best explained it – TRIMESTER CALCULATOR – and looked at the areas of;

Development – uses actual developmental stages to divide up a pregnancy.

  • LMP to 11 weeks 6 days
  • 12 weeks to 26 weeks 6 days
  • 27 weeks to birth

Gestation – takes the 40 weeks of gestation and divide by three.

  • LMP to 13 weeks 2 days
  • 13 weeks 3 days to 26 weeks 5 days
  • 26 weeks 6 days to birth

Conception – takes the 38 weeks of post conception development, divides by three, and then adds two weeks.

  • LMP to 14 weeks 4 days
  • 14 weeks 5 days to 27 weeks 2 days
  • 27 weeks 3 days to birth

These two images also take a crack at dividing it up:


I found that it really doesn’t matter exactly when first trimester begins or ends… Most health care providers actually talk about your pregnancy in weeks, rather than months or trimesters… and this carries forward into your little one’s age also, so getting used to weeks is a good thing 🙂

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