How is a due date calculated?

Around about 1850 a German obstetrician called Franz Naegele came up with a formula to estimate the due date of a baby and it’s the same formula still used in clinical practice today. Based on Naegele’s rule, 9 months + 7 days are added to the first day of your last menstrual period (LMP), which equals 280 days. Nowadays the majority of people in high resource countries, like the UK, will have an ultrasound scan at around 12 weeks and based on the measurements of the fetus and your LMP, a due date will be calculated. It’s worth noting that this due date might differ to the one you had worked out based on the first day of your LMP - and this difference can have implications for the care you’re offered and when you’re offered it. And if you know anything about us, you’ll know we’re all about choice! So a gentle reminder that it is your choice to have a 12 week scan or not.

The historian Lucy Inglis in her book, Born, notes that long before Naegele, it is documented by Jewish rabbinical teachings that 10 lunar months was used to estimate roughly when a baby would be born and that the tenth month was thought to be the ideal time for birth - which made for a slightly longer gestational length that Naegele’s rule. Women of course probably used the moon cycles long before the rabbinical teachings put pen to paper, however much of women’s history remains undocumented, with information being passed down orally.

Anecdotally, some people in the birth world use the word ‘guess date’ instead of due date - the simple change in language acknowledges the unlikelihood that your baby will be born on that very specific date given by a scan or your LMP. It may also be helpful to reframe for yourself - instead of putting a lot of pressure on a single date, you can know that your baby could come at some point a couple of weeks either side of it. That being said, everyone and their mum will ask you when your due date is, and it’s your choice if you wish to share when you’ll be 40 weeks or if you prefer to keep it vague. An advantage to keeping it vague is that you might be less bombarded with messages asking if the baby has come if people don’t actually know your guess date.

How accurate is your due date actually?

The estimated due date that’s calculated is 40 weeks. In reality only about 5% of babies are actually born at 40 weeks on the dot - most babies are born between a 5 week window between 37 and 42 weeks. And there are of course babies that are born prematurely before 37 weeks and those that are born after 42 weeks. The UK NICE guidelines state that 99% of women will have gone into spontaneous labour by 42 weeks of pregnancy.

Given the huge variations in us humans, it doesn’t really make sense that we would all carry our babies for exactly the same length of time. There is evidence to suggest that both your family history, and the length of any previous pregnancies play a role in the length of your pregnancy. If your mother or sister had longer than 40 week gestations, there’s a good chance you will too.

Neither your LMP or an ultrasound can be 100% certain what specific date you’ll give birth - so you should see it more as a rough estimate or a timeframe within which your baby is likely to be born.

What does "overdue" really mean?

Because so much emphasis is placed on the estimated due date, it’s not unreasonable that you might think yourself ‘overdue’ if you go past it - or indeed that your baby was ‘early’ if they come before it. It’s important to remember the tiny statistic of roughly only 5% of babies actually being born on their due date and that a baby born anytime between 37 - 42 weeks is completely normal.

Pregnancies that last beyond 42 weeks are called ‘post-term’ and in the context of induction of labour, you might hear your pregnancy being referred to as ‘post-dates’ if your pregnancy goes beyond 40 weeks. It’s also worth saying that a pregnancy going beyond 42 weeks isn’t abnormal - there will always be some babies that are born outside of the 5 week window considered ‘term’.

The pressure that comes with a due date and where it leads

There has been a big shift in the UK maternity system (and elsewhere) when it comes to offering induction of labour. NICE guidelines now recommend offering women and birthing people membrane sweeps (a form of induction) from 39 weeks and to discuss more formal methods of induction from 41 weeks, in an otherwise uncomplicated pregnancy. Guidance used to suggest offering formal methods of induction at 41 weeks and 5 days, but following the publication of the Swedish SWEPIS Trial which was comparing induction of labour at 41+0 weeks with induction of labour at 42 weeks, the advice in the UK changed - despite there being flaws noted in the research. Another study that has influenced rates of induction around the world is the ARRIVE trial, despite being another heavily flawed piece of research.

The rationale for induction is to reduce the risk of stillbirth - which whilst yes, is higher at 42 weeks but is still very low. Research from 2019 looking at stillbirth rates in uncomplicated pregnancies showed the following risks:

• 0.33 per 1000 at 40 weeks
• 0.80 per 1000 at 41 weeks
• 0.88 per 1000 at 42+0 - 42+6 weeks

So the risk of stillbirth is higher between 42+0 and 42+6 weeks compared to 40 weeks - but the overall risk of stillbirth at each of the gestations above is still less than 1%. This is not intended to minimise the impact that stillbirth has on women, birthing people and their families - every stillbirth is a tragic loss. Unfortunately the stillbirth rate is never going to be 0%.

We don’t believe it’s fair that you may be pressured into interventions such as induction of labour or caesarean birth based on a <1% risk, when the risks associated with induction and caesarean birth are higher. We know that in practice (from what we have seen and what we hear from women) the risk of stillbirth can be overinflated and the potential substantial risks for you and baby of induction or caesarean birth can be brushed over by some clinicians. This is the ‘all that matters is a healthy baby’ mindset.

What are your choices if you go past your due date?

If you’re receiving care from the NHS you’ll be offered membrane sweeps from 39 weeks - it’s your choice if you want to accept these or not. At 41 weeks you’ll be recommended to have an induction of labour. You might choose to accept these interventions, or you might choose to wait. You might have an idea in your mind about what you feel your threshold for waiting is. If you decline induction or caesarean birth, you’ll generally be offered more monitoring (for example an ultrasound scan or CTG monitoring of your baby’s heartbeat) - it’s worth noting that there isn’t evidence to support that this extra monitoring improves outcomes for you or your baby, but some people find it reassuring nonetheless. Again, it’s your choice if you want to have any additional monitoring and you shouldn’t be pressured into it. Some women will choose to have a planned caesarean birth instead of having an induction of labour if they reach a certain point in their pregnancy.

How Juno Midwives support you around your due date

We have really in depth antenatal appointments with our clients, where you see the same midwife and have an abundance of time to discuss your pregnancy and your wishes around your birth. We discuss the local and national guidelines with you and explain the evidence (or sometimes, lack of evidence!) for these recommendations. These are often chats that are naturally woven in during your pregnancy - rather than us dropping it on you at 40 weeks. Some of our clients know from the off that they will be happy to wait for their baby to arrive if all is well. Others have a specific gestational cut-off point in their mind. We’ll support you throughout your pregnancy to develop self-trust and to make the choice that feels right for you.

It’s your body and your baby - and you only get to give birth to this baby once. Our role is to support you, provide you with the evidence and the information you need to make an informed choice and then to support whatever choice you make.