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"When is my due date?" This may very well be the first question a woman who just found she is expecting a baby asks or wonders about.

Yet, only four percent of women some of whom are induced give birth on that date, and only 70 percent actually delivers within 10 days of their official estimated due date.  

So, how long does pregnancy truly last? Did the women who didn't deliver anywhere close to their due date but who didn't have preemies get the date of conception wrong or did the ultrasound give the wrong info? This is what a study team from the US National Institute of Environmental Health Sciences set to find out.

They got a bunch of women together, who were all trying to conceive naturally. Daily urine samples measures their hormone levels so the researchers could determine exactly when ovulation occurred, and after that the implantation of a fertilized egg. The findings? Well, they led the research team to question how helpful estimated due dates are at all, because pregnancy naturally turned out to vary in length quite a bit. The researchers were surprised by their own findings.

They found out that the average gestational length (from ovulation to birth) among the women that participated in the study was 268 days, after excluding obviously premature births. That is just over 38 weeks, which is indeed what we consider to be full-term. But gestational length varied by as much as 37 days naturally. What's really, really interesting is that the researchers who published their findings in the journal Human Reproduction discovered that the embryos that took longer to implant turned out to be babies that gestated for longer!

Dr Anne Marie Djukic, the study's lead researcher, said:

"We were a bit surprised by this finding. We know that length of gestation varies among women, but some part of that variation has always been attributed to errors in the assignment of gestational age. Our measure of length of gestation does not include these sources of error, and yet there is still five weeks of variability. It's fascinating."

The rest of the findings were not shocking, and basically in line with what was already known about gestational length:

  • Older mothers tend to have longer pregnancies. We already knew that moms are more likely to "go overdue" when they are older, but this may be a completely natural phenomenon.
  • The same mother is likely to have pregnancies of a very similar length, meaning the variations in possible pregnancy length could be individual and not related to the number of children one has.
  • A mother's weight and gestational length are somehow connected.

Dr Djukic concluded:

"I think the best that can be said is that natural variability may be greater than we have previously thought and, if that is true, clinicians may want to keep that in mind when trying to decide whether to intervene on a pregnancy."

In other words, the natural birth crows that goes on about "variations of normal" so often and is ridiculed for it by certain blogging OBGYNs may be right after all. A lack of medical intervention, or should we call that expectant management, may be the right approach because yes, women's bodies might know best after all.

For now, there is evidence that leads doctors to conclude that pregnant women should be delivered once they reach 42 weeks. This is the point at which the incidence of stillbirth goes up something that has been linked to the possibility that the placenta comes to the end of its natural abilities, and stops working properly.

It would be interesting to see how this study will influence clinical practice and perhaps trigger further research. In the meantime, perhaps we should be more like Kate Middleton and only give approximate answers when people ask us about our due date. We could point out that this varies by as much as 37 days and that it's hard to say. "I might be due in August or September" might be a more accurate answer than, "I am due on September 2", after all. It also means that you can relax a little, and not worry about induction too soon. What are your thoughts? Please leave a comment if you would like to share.

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