Why Only 3-5% of Babies Are Born On Their ‘Due Date’

Why Only 3-5% of Babies Are Born On Their 'Due Date'

It’s rare for a pregnant woman to not know her due date (or ‘guess date’ as we like to call it at BellyBelly).

While she might not tell you what it is – and she shouldn’t feel obliged to – there is a particular day in the future her mind is focused on most of the time.

Most women find it hard not to become fixated on their due date, even those who aren’t really fussed when their baby is born.

For one thing, doctors tend to get antsy when babies aren’t born when they are due. And of course, we just want to meet the little person in there!

Due dates are funny things though. Around 80% of babies actually arrive in the 2 weeks before and the 2 weeks after the day they are predicted to.

Which means waiting for your baby’s due date is an endurance test most of us don’t really want to be subjected to.

The Magical Due Date

When your first period is missed or you have a positive pregnancy test, it’s common to see your health care provider who will calculate your estimated due date (EDD).

The predicated or estimated date of birth is based on Naegele’s Rule, which is to add 280 days (or 40 weeks) from the first date of the last menstrual period.

The theory that gestation lasts 40 weeks was originally thought up by a botanist in 1744, who believed pregnancy lasted 10 lunar months.

In 1812, German obstetrician Franz Naegele publicised this formula. Since then, this method of calculating the estimated due date has become the accepted norm, which is now centuries old.

The Excitement Of The Date That Makes All Other Dates Seem Irrelevant

Once you have been told your magic due date, you might even decide to have a dating scan. This is an ultrasound which measures the length of your teeny tiny baby and the size of the gestational sac. If these numbers don’t match up with what your last menstrual period has predicted, you might be given a new due date!

Either way, you’re excited! You calculate how long it is until you will share your news (or maybe you already have!), or when it’s likely you’ll stop feeling sick. There are so many things to plan, like the nursery, choosing names, deciding where to give birth…it feels like there isn’t enough time to get everything done!

You are probably already curious as to what your baby is up to in there! You can follow your baby’s development every week, in BellyBelly’s awesome Pregnancy Week By Week series. Your body starts changing as your pregnancy develops – and these changes can be amazing and sometimes a little odd!

Is This It? Wait… Now?

The lead up to labour can be a really exciting but frustrating time, especially if it’s your first baby. Not really knowing what you’re looking for but paying attention to every twinge and ache keeps you in a state of high alert! See our signs of labour article if you’re curious.

Prelabour can start and stop too, so you might have a few days, nights or even weeks of feeling like something is happening, but it all fizzles out. This can definitely lower your happy mood and look out those around you if you’re not getting enough sleep. At this point, you definitely need to read our article, 8 tips for a low stress early labour at home.

Not to mention the amount of people who are just as interested as you are to see if the baby is on his or her way. Texts, emails, Facebook messages… you can’t escape the questions and advice: ‘anything happening?’ ‘come on baby, we want to meet you’ ‘ what does the doctor say?’ or even the dreadded, ‘surely they can’t just induce you by now?’

You’re not ungrateful that everyone is excited too, except it’s you that’s having to physically deal with the waiting. Your back aches, your feet are swollen, you can’t go more than walking distance from a bathroom and honestly, what is this baby waiting for?

The Day Is Here!

You might wake up with a little knowing smile on your face today. You’re sure your little sweet one is going to arrive on this auspicious day – because this is the end of the 40 (long) weeks of pregnancy. You might have one or two of these thoughts enter your head on your due date.

Maybe you’ve been warned a few times or read online not to pin too many hopes on this day being your baby’s birthday. After all, only 3-5% of babies are born on their due date. Which, when you think about it, isn’t a lot. And why is gestation only 40 weeks anyway, not say 38 or 42?

One of the glaring flaws in Naegele’s Rule is that strictly speaking, a lunar month is actually 29.53 days.

So 10 lunar months works out to be 295 days – 15 days or two weeks longer than the 280 days gestation length we are currently told is normal.

If leaving nature to do her thing, first time mothers will tend to go into labour around 41 weeks. Women who have already had babies before tend to give birth around 40 weeks and 3 days.

The Date… It Has… Been and Gone *Sad Face*

So the estimated due date your doctor smilingly handed you has passed. The messages and phone calls are increasing… and frankly starting to really annoy you. You can’t be bothered getting out of the house to avoid people asking when you are ‘due’ incase you shout at them.

Another massive flaw in the current method of calculating a woman’s due date is the assumption all women have a 28 day menstrual cycle.

Some women have longer or shorter cycles and not all ovulate mid cycle on day 14. Women who have long cycles would naturally be more likely to gestate longer than the given 280 days.

So your doctor or midwife has checked you and your baby and everything is going fine. Maybe you’ve tentatively enquired as to when labour might be likely to show up, and your care provider sweetly says, “any time now!”

Which is not really helpful. Because you’re over the waiting. And not being able to get off the couch. And your back really hurts. You can’t sleep because you are so uncomfortable. Your emotions are all over the place, angry one minute, crying the next.

Not to mention, you’re worried about being pregnant forever. What if your body won’t understand what it has to do and labour doesn’t start? What if something is wrong?

What The Experts Say and Induction of Labour

Many health experts agree that letting labour begin on it’s own is best for women and babies, and it’s best not to interfere before 42 weeks. There are plenty of reasons why you should give your baby 40+ weeks of pregnancy.

However, in many hospitals, there is a policy to induce labour once a pregnancy has passed 41 weeks.

You may have heard an induction of labour is awful and want to avoid that at all costs. Perhaps your ‘up-to-now’ easygoing doctor has suggested booking an induction date ‘just in case’. It’s a good idea to know what the risks and benefits of induction are before agreeing to having your labour started artificially.

How’s a woman supposed to go into labour with all this pressure?!

Be prepared to have some extra monitoring if you pass 41 weeks, just to make sure baby is happy. Unless your healthcare provider can provide you with evidence that you or your baby are unwell or if complications arise, rest assured labour will begin when your baby is ready to make the transition from womb to world.

Whether you are one of the rare few who give birth exactly on your due date, in the weeks before or after, it doesn’t make any difference in the end. You will be happy to meet your new baby whenever he or she arrives.

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Sam McCulloch enjoyed talking so much about birth she decided to become a birth educator and doula, supporting parents in making informed choices about their birth experience. In her spare time she writes novels. She is mother to three beautiful little humans.

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