Let's Talk About;
Due Dates and when your baby will be born.

Do you remember when you first worked it out? Did you use a calendar? An app? An online calculator? Or, was it one of those paper wheel thingies that the midwife used? I’m talking about your baby’s due date. Or rather their estimated due date - EDD for short.

For me, 11th September and 1st June are dates etched on my brain from when I was carrying my two children. Spoiler alert: neither of my children were born on their due date as it turned out. But alongside their actual birth dates these dates feel special to me and I remember them.

You may well have the date circled in thick red pen on your calendar or starred in your diary. The date will also be a significant piece of information that will be used by your care team. Whilst this date is only an ‘estimated’ due date you may well find that your care team (and in fact other people in your life) treat it more as an ‘expected’ due date. They may behave as if this date is more etched in stone than pencilled in.

So, is it an important number? But have you ever stopped and asked how it is worked out? Is it accurate? Is it useful? And does it really serve us?

GETTING TO THE NUMBER

Your EDD is calculated from one key piece of information; the first day of your last menstrual period (LMP). Your EDD will generally be established as 40 weeks, or 280 days on from that date.

You may find that this date is adjusted at your 12 week scan when the sonographer will be able to take several measurements from your growing baby to help with gauging gestation. Typically, this date from the scan will ‘override’ the previously given date. Sometimes this is only a day or so. But it can be significantly more.

Now, you may already have spotted some flaws in the arbitrary formula used to calculate your EDD.
• It assumes a regular as clockwork 28 day menstrual cycle where ovulation occurs on day 14. I’m sure I don’t need to tell you that for many women this textbook kind of cycle isn’t a reality.
• It assumes that you keep track of or at least remember the timing of your periods and can therefore pinpoint the first day of your last cycle with ease. If you’ve not been actively trying to get pregnant this may be ambitious.
• It assumes that all babies are ‘due’ (i.e ready to be born) after the same amount of time in the womb. As if humans are an exact science.

OK. WHERE DID THIS METHOD COME FROM?

Where does this This ‘LMP + 280’ formula (as it’s sometimes referred to) come from? To answer this question we need to take a step back in time. You’d be forgiven for assuming that this method is the result of modern scientific study and rigorous data analysis. Afraid not! In fact, the formula we use is called ‘Negale’s Rule.’

Franz Negale, was a German Obstetrician. He published his theory of LMP + 280 days in the early 1800s and ever since it's been adopted as the primary and accepted method for establishing a baby's due date. Negale’s work was based on some previous work by a Botanist called Heerman Boerhaave. His calculation was based in part on Biblical scripture which suggests human gestation is ‘10 lunar months’ as established by looking at the dates relating to the conception and birth of Jesus.

His theory was accepted and adopted. Whilst there are a number of slight variations to Naegele's Rule which have come up over the years, Negale’s Rule, now over 200 years old, seems to have staying power.

Maybe we can agree that the method for working out your baby’s estimated due date has some flaws. It’s based on a theory which lacks scientific rigour and it makes a number of assumptions and generalisations about your body and your baby.

DOES IT EVEN MATTER?

You may well be asking why this date matters. Especially as it seems to be commonly acknowledged and generally accepted that most babies are not in fact born on their dues dates. In fact, it’s not unusual to find that several weeks before your due date you start getting enquiries from well meaning family and friends as to whether you ‘feel any movement yet’? And heaven forbid you do past your due date! Some babies are born before this magic date is reached, others some time afterwards. Take a poll amongst those who have birthed before and I’d guess that you’ll get a wide range of results as to when babies made their arrival as well as opinions relating to how accurate these dates are.

The data backs up this generally accepted fact too. We know that only 4% of babies are born on their EDD. Yup. a relatively small number given how much weight is given to this simple set of numbers.

So why does it matter? Why am I dedicating a blog post to this topic? Here are my thoughts:

• Your EDD will be the key piece of information for establishing when you will see your care team. This is because appointments are offered at regular intervals based on your week of pregnancy.
• It follows therefore that you will also be offered scans and other screening tests for you and your baby based upon your estimated due date. Results of these tests can have an impact on the care you may be offered so doing tests and screening at the appropriate time may be important.
• Your baby’s growth and that of your expanding abdomen will be monitored and assessed based on what is considered to be ‘normal’ for the gestational age. From this we get the labels ‘small for gestational age (SGA) and ‘large for gestational age (LGA) when referring to the estimated size of your baby - both of these labels have the potential to impact on things which may be offered or recommended during your pregnancy.
• Should you have any conditions or complications where an early birth is recommended (either by induction or scheduled caesarean) a week or so inaccuracy either way on your EDD can mean you baby who may have been scheduled to be born at 38 weeks is in fact born as early as 36.5 weeks or later at 39.5 weeks.
• Once you get to 40 weeks (and sometimes even before then) you may well hear your care providers offering you an induction of labour. This offer is usually accompanied with the primary reasoning being ‘to prevent you going ‘overdue’. If induction is not something you wish for, this ‘offer’ can be a cause for concern and worry - which in itself isn’t the optimal emotional state for encouraging labour to begin spontaneously.

WHAT ABOUT ASSISTED CONCEPTION?

All of the above is written with the assumption that your baby has been conceived without assistance. Of course, we know that many pregnancies are supported with forms of assisted conception. For those parents-to-be they may well know the exact date of conception as they know the date of their embryo transfer and the age of the embryos upon transfer, for example.

Whilst pregnancies which have been assisted tend to have slightly different care pathways and are generally monitored more closely given some specific risk factors (including premature birth) your EDD will generally be assumed by your caregiver to be the same as if you had conceived without assistance. At the very least this can be frustrating for the parent who knows the date (and sometimes the very time and place!) that their baby was conceived.

OK, SO WHEN IS MY BABY MOST LIKELY TO BE BORN?

The good news is that as far as I’m aware no one has ever been pregnant forever! Despite how it can feel in those last weeks and days when you may be feeling heavy, tired and generally fed up of waiting for your bundle of joy - you will give birth. As to when you birth- no one can offer you any guarantees I’m afraid. Perhaps it may be helpful to consider your baby’s arrival as being a ‘season’. Babies, like apples, ripen over time rather than being born when the timer (of which they are of course totally oblivious) goes ‘ping’.

What we do have is data. We have the benefit of modern research and research techniques to offer us insight and some clarity. One of my go-to places for a breakdown of the evidence on this kind of thing is Evidence Based Birth who publish a wide range of resources for parents and professionals alike. They cover a lot and they help unpick and shine a light on the evidence surrounding a number of pregnancy and birth related topics. (when you’re done here I’d recommend checking them out).

When looking at what they have to offer on the topic of due dates they refer to the Jukic et al., 2013 study. The research looked at gestational length and birth dates as measured from LMP. As the study women even before conception the researchers knew the exact days that the participants ovulated, when they conceived, and even when their pregnancies implanted. The data they gathered is considered to be reliable and useful despite it being a relatively small sample size.

This study suggests that the median time for a pregnancy (as measured from LMP) was
40 weeks and 5 days.

The length of pregnancy ranged from 36 weeks and 6 days to one person who gave birth 45 weeks and 6 days after the last menstrual period. (Which admittedly sounds like a really long time but that particular person actually gave birth 40 weeks and 4 days after her known ovulation date.)

If you prefer a percentage then the headlines are this:

10% gave birth by 38 weeks and 5 days after the LMP
25% gave birth by 39 weeks and 5 days after the LMP
50% gave birth by 40 weeks and 5 days after the LMP
75% gave birth by 41 weeks and 2 days after the LMP
90% gave birth by 44 weeks and zero days after the LMP

In a nutshell:
For first time mothers about half will go into labor spontaneously by 40 weeks and 5 days.
For mothers who have birthed before half will go into labour spontaneously by 40 weeks and 3 days. The other half will not.

This data appears to suggest that 40 weeks gestation as standard could be considered as slightly short.

OK. THA'S A LOT TO TAKE IN. CAN YOU SUMMARISE?

Ok, let’s wrap it up.

• Much may be made of your due date. And of course there are many practical reasons to establish an estimated due date for your baby’s birth. However, perhaps problems arise when we lose sight of the word ‘estimated’.

• We can hopefully acknowledge that the method for establishing an EDD is at best clunky and at worst outdated.

• We know that EDD can have an impact on the care that you are offered. And that towards the end of pregnancy the EDD can become a more pressing concern as it will be used as a measure for when induction of labour is offered to you.

• The evidence we have appears to suggest that the traditional 40 weeks gestation period may be slightly short.

• When we look at the median gestation of 40 weeks and 5 days we know that 50% of women will give birth by then and 50% will not.

• You are most likely to give birth between 37-42 weeks of gestation.

• You will not be pregnant forever, I promise.

I’d like to end with a piece of wisdom that a member of my one care team offered me when I was pregnant with my daughter. She wisely said:

‘Babies are not born early. Babies are not born late. They are ALWAYS reliably born on their birthdays.’ and that much we CAN guarantee.

Much love

Becca
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