
This is a constant source of glee to me when I am not pitching fits about how counter intuitive it is, but really, who came up with the mathematical ciphering that we use to discuss a pregnancy? Seriously, I haven't seen anyone go so far to obscure real understanding in the name of science since the Great and Mysterious Oz.
Right off the bat, what are we talking about? The way I see it, we've got a build-up to delivery on our hands. Due dates are not an exact science (believe me, this much I know!)but would not a count-down be more appropriate? Imagine the launch of a space shuttle or the dropping of the ball at Times Square on New Year's Eve going up instead of counting down. Didn't that suck?
Okay, OK. I know that the build up is the only way it worked in the past, but how many mommies (in the States, at least) schedule their delivery date to the 15-minute block in this day and age? Google it, people, I'm making a point about misleading jargon having no place in everyday communication... people can do whatever they want with their bodies!
It's not like it needs discussion at all. If you say it's the 40th week and I say it's really 38... that won't stop the world a'turnin', and it won't matter one bit to the so called unborn child, either. It's just that doctors talk to people about the babies they'll have in a way that clouds the a-ha! People don't even know it. I didn't. You see somebody with a belly bump, or hear that a friend is expecting and the knee-jerk utterance is, "How many weeks is it?" Then you hear the answer and by that time your mind is wandering in twenty-five thousand different directions.

So, here's what we know: we're talking about a pregnancy and weeks is the unit of analysis. Good. Now there is a little matter of Gestational Age (what doctors go by, and hence, what we all are using) and there's Embryonic Age. OK, no big deal. But what are you concerned with?
I think the natural answer is that we are interested in the age of the baby. Isn't that what you're asking?
Could you ever see yourself walking up to a woman and asking her, "So, um, when was the last time you menstruated? C'mon, c'mon, I'm talkin' to you here! When was your last period? Spit it out, sweetheart, when was it?!"
Hey, that's just rude!
But that's what doctors have us doing, you know?
Gestational Age is the number of weeks since the beginning of the final menstrual cycle before conception, while Embryonic Age is the age of the embryo/fetus since conception. So, if we're gonna insist on using weeks to discuss pregnancy here's the formula:
Let x equal the number of weeks of the pregnancy.
If Gestational Age is x weeks,
the age of the pregnancy is x-1,
the Embryonic Age is x-2, and
the actual age in terms of Embryonic age is x-3.
Um, where x is weeks,remember? Got it? Good!
So, when we say we're in the 6th week...
are you ready?
That's what the doctors tell us and it means that fetus is 3 weeks past conception.
WHY, why, oh why?!

I'm just assuming that it's an inside joke by doctors to make each other crack up once we've left their offices. And I know doctors. I just spent a week with a doctor who'd attended the delivery of countless babies. One of my oldest friends, actually. I confided in him that I was nervous about my ability to hold it together during delivery and so I asked him what I could expect. Do a lot of fathers get woozy, do any of them pass out, should I expect a rush of heroic fervor, WHAT?!?

He told me a lot of moms poop during delivery!
Thanks a lot, Doctor!
But there's more funky pregnancy math to unearth:
Here, it's fun!
Our favorite baby book puts it this way: "You can calculate your due date by counting 280 days from the first day of the bleeding of your last period" and it continues that you could also "count back three months from the date of your last period and add seven days" and then substitute the following year (obviously!) if the fisrt method isn't involved enough for you.
Please bear in mind that only 5% of women will deliver "on their actual due date" when you undertake to begin these computations!

Awesome!
But before all this is an issue there's the pre-pregnancy math! For example, it seems so simple that one egg+ one sperm = one fertilization. And that narrative is vastly more common than any other fertilization scenario, although we've all heard of the outliers. What leads up to that coupling, however, is a pretty daunting presentation of numbers.
Let us hop right to it, then. If the pickle is a little girl in there, then she'd have more eggs now than any time in her future. That is to say that by 5 month gestational age, a femal fetus will have produced 6.8 million eggs. At the time of birth female babies will have only 2 million eggs. Don't ask me where they are going, either... obviously the overwhelming majority of babies are born pre pubescent (Benjamin Button, aside). But even so, by the onset of puberty, a female human will only have an average of 400 thousand eggs left, and that number (theoretically) should be decreasing at a rate of roughly one egg per 28 days.
Now dudes, dudes on the other hand are playing an equally baffling numbers game that goes on its own weird ride from mind-boggling hugeness to that one + one = one outcome.
Each male deposit (I know, I would've used 'serving' too if I were the original source of this material!) contains up to 350 million sperm. Every time, to a man, like clock-work. Right. And of that number, say 200 million may reach the fallopian tube where the egg is (it's a "don't take the wrong exit" scenario for our trepidatious little swimmers) and then I can only imagine they do "Scissors- Paper- Rock" for the honor of fertilization. I don't know, maybe they 'call' it at some point in the vas deferens.
The point is that we're talking about one pickle who ought to be delivered in a party store and immediately buy some lotto tickets. Go figure the odds: one egg of a possible 6.8 million joining up with one of (I mean
countless!) sperm and then snap-crackle-pop, you know the rest, magic!

So there, we all know smart is sexy. And I think we've just used math to prove the converse as well...
OK, class dismissed, quiz next Friday!