Due dates by Dr Bianca Bryce

I spoke in my last post about still being pregnant after your due date.  Although it makes logical sense that you would have a baby by (or even on) your due date, dates for babies are not like expiry dates on food. It’s a bit like the pirate’s code – that bit in Pirates of the Caribbean when Barbosa tells Elizabeth Swan “they’re more what you’d call guidelines, than actual rules” (insert salty piratical voice here).  It’s called an estimated due date for a reason.

Even if you religiously track your menstrual cycle, and you have a true 28 day cycle (and seriously, how many of us can honestly tick both of those boxes?), or you know exactly the day of conception, there are too many variables.  Sure, you know when you had sex, but did you know that sperm have the ability to squirrel away in the vagina and wait up to 5 days before making a break for it when ovulation happens?  And you may know your cycle back to front, but not everyone ovulates on day 14 like the textbook says, so who knows when that little egg was released and fertilized?  Or maybe you had a dating scan, so that must be accurate right?  Remember that you’re measuring something that is usually less than a centimetre in size, and making predictions based on the size is fraught with difficulty – what if you are a millimetre out?  Where does that put your due date?  And even if all this works exactly as it should, who’s to say that every one of us is meant to gestate for 280 days – part of being human is that we are different from each other in so many millions of ways – why would all our pregnancies be the same?


Of course it makes sense that they aren’t.  A recent study, which apparently “pinpointed the moment of fertilization”, still found that there was a more than 5 week window in which babies were born.  And there are other statistics that tell the same story.  Only about 5% of babies are actually born on their estimated due date.  Generally, less than 60% of women have their babies on or before their due date.  There seems to be emerging evidence that women from different racial backgrounds may have slightly different average lengths of pregnancy.  Different babies from the same mother will be born at slightly different gestations.  Midwives and doctors have long worked under the assumption that it is normal to have a baby between 37 and 42 weeks gestation.  We still don’t really understand what it is that makes labour begin, so why do we pretend that we know when it should happen?

This is not to say that you shouldn’t have faith in your body, or faith in the amazing technology that we are lucky enough to have access to.  But don’t put too much energy into your due date – it is a guide, that’s all.  Trust your body and your baby to do what they need, when they need.  And you can also trust that your caregivers are very unlikely to let you stay pregnant too long…but that’s a story for another day 🙂


(This article contains general information only and is not intended to replace advice from a qualified health professional. All information is written from the experience and knowledge of the person writing the post).