Let's Start in the Middle

So I will have some catching up to do as I am at 20 weeks, which is considered halfway through, but I have been encouraged more than once to write about my pregnancy experience. So, as much as any of you can find the humor in it, I am happy to share my adventures on the beginning of what I anticipate will be one of the wildest rides of my life. Parenthood.
For starters, right away I noticed math goes out the window. You miss your period and though my understanding of biology and anatomy makes it clear to me that actual conception happened with ovulation and therefore mid-cycle, which was approximately two weeks prior, positive test at first day missed? and viola! You are 4 weeks pregnant.
Kind of sweet when you are looking at 40 weeks of all sorts of interesting and unexpected experiences, phew! those first two weeks? All things considered, were a breeze.
Now about that 40 weeks gestation, and I acknowledge in relation to the pregnancy of say, an elephant at two years(!) I am feeling okay with human gestation, but how is 40 weeks equivalent to 9 months if generally months are 4 weeks plus a couple days? Do they just say 9 months to make you feel better because 10 months sounds like almost a year? Or is it including that wiggle room of the first two weeks being actually the, um, coital weeks shall we say, and perhaps they rounded the numbers to include the variation on the actually delivery time on the tail end of this grand adventure. Or is it simply that the "old math" no longer applies.
Because then there was that day in class as I was teaching Anatomy, about week 7 of pregnancy (accepting common nomenclature, so yes actually 5 weeks and only just barely beginning to feel funny(funnier) than usual). My sweet students looking up at me with their brows furrowed, the bravest says, "Dr.Jen? I think you made a mistake with the scoring on the midterm". So my own math also went out the window, probably as I was trying to wrap my head around the whole 40 weeks is 9 months thing. Thankfully my usually more than adequate math skills were only offline for a couple weeks, but I actually could write the formula for what I was trying to do, which was toss out a couple questions that more than half the class missed, put it over the new total and give extra credit to the students who had gotten those right. Somehow I had knocked everyone's points down, and I just couldn't confidently reconcile the math! I actually ended up emailing one of the students privately that evening and asking him to confirm what I thought was the solution (add two points to everyone's score).
Seems so simple now!
So on from addition & subtraction, how about algorithms such as~If I buy these maternity pants that fit now, will they really stretch enough to fit four months from now? Is this even possible?! I ask the lady at the store? She assured me it was. I'll let you know.