Once our children are born, it seems we have infinite patience for the great diversity and variety of growth and development among them. One gets teeth at this age, another a month later. My first walked at 9 months, my 2nd at a year and so on. We even seem to take it all in stride that one baby weighs in at 6 lbs. 7 ounces while another at 9 lbs. 2 ounces. Why then are we so quick to believe that millions of babies will somehow, magically be ready to be born at precisely the same time in their gestation?

Try though we might, none of us can quite believe that we don't have a due date. EDD actually stands for estimated date of delivery, not due date. Personally I much prefer EDB (estimated date of birth) and certainly not the original term EDC (estimated date of confinement). No matter what you call that date on the calendar, it is nothing more than a formula derived from statistical averages which says that sometime within a range of 4-5 weeks your baby will probably be be born. Smack dab in the middle of that range is one of days on which the labor may start. Yet when it comes to dates in our life, few take on more significance than this one.

As absurd as this sounds, we live in a culture where giving birth beyond the 'due date' has become pathological. Normal physiological gestation in a human is 37 to 42 weeks. The World Health Organization, the American College of Obstetricians & Gynecologists, the International Federation of Gynecology & Obstetrics, and the Society of Obstetricians and Gynaecologists of Canada all define 'post term' pregnancy as continuing beyond 42 weeks. Translated, this means that before 42 weeks is not post term. (Past due, post dates and post term are phrases used to discuss this issue, but the current preferred terminology is 'post term'.)

In 1998 approximately 18% of US pregnancies lasted beyond 41 weeks and of those 10% went past 42 weeks. By 2005 these figures were down to 14% and 6% respectively. Multiple factors are contributing to what is called this 'shift to the left'. One positive change probably derives from increased accuracy in the dating of pregnancies through earlier ultrasounds, thus eliminating 'pseudo' post term pregnancies. But it is likely the main reason for the decrease in births beyond the EDB is the increase in technological deliveries (inductions and cesareans), many driven by concerns for the increased risks of pregnancy in the post term period. Indeed, in Listening to Mothers II, 'caregiver concern that mother was overdue' was reported by mothers as the number one reason for inductions of labor. In addition, the overall increase in elective and interventive births for all reasons has precluded more and more pregnancies from reaching their 40th, 41st and 42nd weeks.

For the woman whose pregnancy continues well beyond her EDB, the issues surrounding post term pregnancy are important. This can be a scary time. We are told that the risks of fetal deaths increase the farther past our due dates we go and therefore (at some point) it becomes safer to induce labor than to wait. But what is that risk? For which mothers and babies? When? What is the evidence? And... does it apply to me?

On the one hand, who really wants to think about or discuss the issue of babies potentially dying when you're a healthy mom with a normal pregnancy? But let's talk about it. Let's talk about it because not knowing and understanding this issue makes it much harder to make informed choices when and if the time comes to choose what to do once someone's pregnancy moves past 'half-time' (your EDB).

To this end, over the next few weeks, Science and Sensibility will publish a series of posts on Post-Term Pregnancy to explore and discuss the following:

• 'Am I a Ticking Time Bomb or What?' - Unraveling the Evidence Behind Post Term Pregnancy as a Time of Danger
• 'Watchful Waiting or Induce?' - The Complex Issues of What to Do When Baby Hasn't Come

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