The due date mystery

Source

Who would have thought that something as deceptively simple as a due date calculation would be so confusing to so many people?

I've always known that pregnancy was roughly 9 months long, but like many other people wrongly assumed that the 9 months was calculated from the date of conception. Imagine my surprise at  my first OB appointment! She asked me for the first date of my last period and once I told her, she looked at a chart and said "ok your due date is April 16th..." Just like that..., I looked at her face to see if maybe she was joking but nope she was dead serious; that was my due date and the tiny chart in her hand had  helped her figure it out just like that.  And there began my education on how pregnancy really is calculated in the medical world. The date of conception is not factored in at all. It is calucated as being 40 weeks from the first day of your last period. 

It took me a minute (or two) to adjust my mind to this mysterious method of calculating the due date, so basically by her calculation I was already 6 weeks pregnant, even though for the last 4.5 weeks I had been blissfully unaware of it. Which means it is impossible to ever be 1 week pregnant or 2 weeks pregnant or even 3 weeks pregnant because in the first 3 weeks of the "40 week pregnancy" you aren't really pregnant yet. Confused yet?

Okay it's really not that confusing once you get over the initial bewildered feeling. Basically doctors have found it more reliable and simpler to follow the 40 week rule and count from the first day of the last period rather than attempt to figure out a conception date.  Either I didnt pay much attention in biology class or my teacher didnt do a good job of explaining it but I had always assumed that conception was a straight forward thing; ie the baby is conceived on same day thae you have sex. But of course I was quite wrong, it can actually take several days for the entire process of fertilization and implantation to be complete. Which means you can have sex on monday but not have "conceived" until friday or saturday and your body still may not feel any different or show any symptoms, or give a positive result on a pregnancy test for  at least another 2 weeks.

For some reason many women assume that 40 weeks = 10 months; this is one of my pregnancy pet peeves. I just don't understand why something so simple is such a point of confusion. I guess it might be because they roughly calculate that each month has 4 weeks, but somehow forget that most months have 30 or 31 days which is actually 4 weeks plus a few days...  Everytime I hear someone talking about how they just realized that pregnancy is actually 10 months, I can't help but cringe!