The So-Called Due Date

If there is one date that your baby is unlikely to arrive, it is on your so-called due date. Approximately 96% of people do not give birth on that date. Odds are, neither will you.

So when do people typically give birth? Where does the putative “due date” come from, and why do we even call it that?

Babies are considered full term anywhere from 37 weeks to 42 weeks gestation. This means that there is a five week window during which your baby is likely to be born. It does not mean that all babies are equally ready at 37 weeks, and just waiting in a queue until labour starts sometime during that time frame, but rather that the majority of babies reach maturation during that span of weeks. Most babies are ready to be born at some point between week 37 and week 42.

Only 4-5% of people give birth on their actual ‘due date’. Approximately 90% of babies will arrive naturally between 37 - 42 weeks gestation. This, of course, means that the other 10% of outlier babies are ready before or after that range. But when we’re looking at averages, the outliers are the exceptions. And there is nothing necessarily wrong with babies that are naturally quicker or slower to gestate.

While births span both sides of the due date regularly, there is a slight difference between first versus subsequent babies. First babies tend to arrive more commonly after the due date. In fact, the average first time parent goes into labour approximately eight days after the due date: a full week! So while some first babies arrive before that elusive date, many more arrive significantly after.

Knowing this, we encourage everyone to take a big step back from the idea of a specific “due date”, to focus more broadly on a due month, or perhaps even a due season. It is most unlikely your baby will arrive on the actual due date. Many babies arrive earlier, and even more arrive later. So for the sake of psychologically preparedness in the case of the former, and to reduce unnecessary concerns in the probable event of the latter, it makes sense to broaden our idea of when the baby might actually arrive. 

If you mistakenly think that the baby is supposed to arrive on one specific date, and then it doesn’t happen, it is easy to feel like something is wrong. When the due date comes and goes, every day that drags by can feel like a week, and every week can feel like a month. Our level of worry and anxiety might rise, and we might make different decisions if we think something is wrong, than we would if we understand that everything is right on track. 

You might notice that people frequently use misleading language around this. People often say “my baby was five days overdue” when the baby was actually five days past the due date. This is not overdue. For a first baby, it is not even average until a whole additional week has gone by. Yet people feel like the passing of the due date means the baby’s birth is delayed: they wrongly think they are “overdue”. The actual definition of “overdue” is after the 42 week mark, not after the so-called “due date”.

So if so few babies actually arrive on the due date, why do we use that number? How did it come to be?

Your “due date” is usually calculated by counting 280 days from the first day of your last menstrual period. This is supposed to equal ten lunar months, or ten 28 day cycles.

Right away, we might notice that this should not apply equally to everyone. Women’s menstrual and fertility cycles are not all twenty-eight days long. Many people have shorter or longer cycles, or irregular cycles. This immediately changes the calculation for many individuals. In fact, one study that contrasted when babies arrived with women’s menstrual cycle lengths, found that longer menstrual cycles are associated with longer pregnancies. If your cycle normally exceeds 28 days, it is common to begin labour later rather than sooner.

The next thing we might notice is that this method does not take into account when we actually conceived. People can be fertile for different lengths of time within a cycle. And ovulation can occur within a variable time frame within a cycle. While some people ovulate on day 14, others ovulate earlier or later. And some people vary from cycle to cycle. This also affects the length of our pregnancies.

Even if we know when we ovulate, conception does not always align exactly with the first fertile day of our cycle. One study that tracked conception (IUI and IVF) found that pregnancy lasts an average of 267 days from conception, which is slightly more accurate than the standard “due date”. But of course, since the standard due date is only 4% accurate, this isn’t saying much. 

So where did this due date calculation come from?

Our current due date model can be traced back to 1744 when a Dutch professor named Hermann Boerhaave proposed a formula based on observing 100 women. Yes, one hundred women! That’s it. He proposed starting with the last menstrual cycle and adding 9 months and 7 days.

This idea didn’t gain much prominence until 1812 when it was revisited and expanded upon by a German professor named Franz Karl Naegele. Unlike Boerhaave, who never specified if you added the 7 days to the first, middle, or final day of the last menstrual period, Naegele used the last day of the cycle in his models. This continued as the accepted way to calculate due dates until the 1900’s, when for some unknown reason, American textbooks switched the calculation to the first day of the last menstrual period. This shortened everyone’s due date in a way that is thoroughly inconsistent with nature, and was never what Naegele intended.

One of the results of this switch is that we now have a misalignment between our expectations of when a baby should arrive, and when a baby is likely to actually arrive. This creates confusion, concern, and according to several studies, an increase in the number of medical inductions. If we believe the baby is late, we are more likely to intervene, even in the absence of other indicators. 

Another approach to determining the due date is using ultrasound technology.

Studies find that ultrasounds performed in early pregnancy are better predictors of baby’s age than ultrasounds in later pregnancy. This is because babies have specific growth markers between 8 and 20 weeks gestation, and tend to be similar in size. Still, even the most accurately timed ultrasounds have a margin of error of 1 - 2 weeks, which is hardly more predictive than Naegele’s due date.

Ultrasounds performed later in pregnancy are even less accurate. They measure the baby, and compare the baby’s size to an ‘average’ sized baby. If your baby is bigger, your due date is assumed to be sooner. If your baby is smaller, your due date is assumed to be later. The problem with this is that babies are naturally different sizes, especially later in pregnancy. Since foetuses between 8 - 16 weeks gestation are more similar in size, any measurable differences can indicate a more reliable measurement of development. But later in pregnancy this is no longer so. Your baby might just naturally be bigger or smaller, with its size having limited relevance to its gestational readiness.

Another point to note here is that all the numbers used are averages, or means. This is the size of all babies studied added together, and then divided by the number of babies. While averages can be very useful when looking at trends, they can also be misleading when looking at individuals. For example, if the parents are both large people, it is common that their baby would also be on the large side. An ultrasound might then reveal a larger-than-average baby, and the due date might be adjusted accordingly, without taking into account the probable genetic disposition to largeness of this particular baby. Size is not always an indicator of development.

Averages are not prescriptions. Averages are not goals. Numbers based on averages should always be assessed in the context of what they are, and weighed against the individual’s specific information. Smaller people tend to produce smaller babies, which is not the same ‘smallness’ as the size of a less developed baby, just as a short adult might measure the same as a tall child, while still having key developmental differences. In fact, averages don’t always represent anyone at all. If I add up the ages of everyone in my house (2 parents and 4 kids) and divide by six, I get a number that represents exactly no one. Averages should never be confused with ideals. 

It is worth noting that the trigger that begins labour is actually your baby. The baby sends hormonal/chemical signals to the mother’s body when they are ready to be born. This stimulates the production of oxytocin, and launches the cascade of hormones that becomes labour and birth. Why does it take some babies longer than others? Because development is unique to the individual. We know that there is a wide range of normal in terms of when people start puberty, when babies grow their first tooth, when babies start to crawl or walk, and all other developmental milestones. Gestation is no different. There is a range of normal; babies simply mature at their own rates.

So when is the actual due date? As far as research can tell, there really is no specific date on which you can expect your baby. Pregnancy typically can last anywhere from 37 to 42 weeks, with the highest concentration of people going into labour between 40 and 41.5 weeks, and some people giving birth even earlier or later than the outer range. Some babies just need more or less time before they are ready to be born.

So instead of focusing on a particular due date, think instead of a due month, or a due season, and know that your baby is unique, and will be ready to be born in their own time.

Stephanie Ondrack has been with the Childbearing Society since 2003, and lives in East Van with her partner, 5 cats, 4 chickens, 2 rats, and 4 kids. None of whom have seen a diaper in well over a decade.