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Due Dates - Helpful Or Not?


When you find out you are pregnant one of the first things you might look up is a due date calculator to know when this baby might be arriving. When you go to your booking appointment with your midwife they will also calculate your due date and then when you have your scan around 12 weeks (the dating scan no less!) they will again calculate a due date.

 

Lots of people will ask when you are due. It dictates your appointments – scans, blood tests – all done at a specific point in your pregnancy.

 

As you get closer to your due date more and more attention is paid to that date. Not necessarily by you but by the people looking after you. If you are having a planned caesarean birth it will usually be done around 39 weeks, if there concerns about you or baby you might be offered induction or caesarean as early as 37 weeks. At 40 weeks you are offered a sweep and some conversations about induction with the goal of having your baby by 42 weeks.

 

But how are due dates calculated? Well there is some science and some not so science behind the different ways.

 

When you have your scan, the sonographer measures something called crown to rump length – the measurement from the top of babies head to their bottom and based on that reading you have a due date. At 12 weeks your baby measures a little over 2 inches.

 

When you or your midwife initially calculated a due date you will have based it on your last period (LMP). This calculation assumes you ovulate at day 14 and have a 28 day cycle. Some online calculators ask for cycle length to try and give a more accurate date, because if you ovulate on day 18 and have a longer cycle then your due date would be a few days later. You then have the added complication that sperm can live in the body for a few days so we can’t really know when fertilisation takes place. You might know the exact date you had sex but that doesn’t mean that is the date of fertilisation. Still with me?

 

Naegele’s rule is the way due dates are calculated based on LMP. You take the first day of your last period, go back 3 months, add 7 days and add a year. Naegele was alive 1778-1851 so this is no modern science method and might not be all that helpful if you have irregular or longer cycles.

 

When I was doing some research on this I came across another calculation Parikh’s formula. You add 9 months to the first day of your last period, add your average cycle length and take away 21. If your maths hasn’t failed you, your due date is about a week later than with Naegele’s formula.

 

In the UK we base our due date on 40 weeks pregnant, and this is the same in many other countries but there are others where due dates are set from 40+3 to 41 weeks (such as Norway and France). This is interesting because on average babies arrive closer to 41 weeks so maybe their expectations are a little more realistic? And maybe Parikh’s formula is a bit better?

 

I see lots of people who are very focused on due date, sometimes with a real expectation that is when baby will arrive when in reality about 4% of babies arrive on their due date! So does it really matter when your due date is anyway?


The language around dates can be a little confusing. We know that babies arrive at anytime but usually ‘term’ is considered anything from 37-42 weeks but sometimes 37-38+6 is early term with 39-41 classed as full term. For a lot of people anything after 40 weeks is overdue but really you aren’t ‘post term’ until 42 weeks. 41-42 is late term. Terminology aside that means that a normal length of pregnancy is 37-42 weeks. That’s a 5 week range. That’s a lot of normal variation and yet we are caught up on one specific date.

 

Due dates are used to schedule routine pregnancy appointments and tests but towards the end of your pregnancy they might be used to plan for birth, especially is your pregnancy is considered higher risk and baby might need to be born earlier or if you are approaching 41-42 weeks.

 

If you are considering an intervention to have your baby sooner make sure you are fully aware of all your options and understand the pros and cons of saying yes and no to them, ask about alternatives and ensure you are have all the information you need to make a decision you are comfortable with.

 

If you are offered an intervention because you approaching 42 weeks this is a helpful read on risks associated with longer pregnancies https://www.aims.org.uk/journal/item/induction-at-term and if you are considering induction I would really recommend Sara Wickham – she has loads of free resources and also a couple of excellent books https://www.sarawickham.com/  

 

So what can you do? Well, firstly don’t worry too much about the due date. Have you noticed it is written everywhere as EDD? ESTIMATED date of delivery.

 

If people start talking about interventions keep in mind the way your due date was calculated. Is there a difference between your scan and your LMP dates or to what you know. Nothing is 100% accurate and given that your baby might want to be there for anything from 37-42 weeks anyway it’s really hard to judge. Although the picture is not complete it is known that the baby releases some chemicals to initiate labour so it is likely that it is up to them when they are ready to be born.

 

So whilst a due date is a nice thing to know, don’t count down to it and if you want to make plans for a massage or dinner out that day you might as well! If you expect to feel some pressure from those around you waiting for baby news baby tell them a due month or that baby will be here by the middle of x month or the end of the month or even just add a couple of weeks on and give that as your due date. Most babies are born by 42 weeks so you’ll escape the ‘any signs yet?’ questions until a bit later.

 

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