gestation is actually 38 weeks in humans, but you are "pregnant" for 40 weeks, because it is based on a 28 cycle assuming you ovulate halfway between.
the extra weeks comes from difference of the day of last period and day of ovulation.
so on day you conceive you are considered 2 weeks pregnant already!
but not many people can tell for sure when they ovulate/d or indeed conceive/d.
if the egg is not fertilized you'll have a period 2 weeks after ovulation. that's a fact and when you have a period you will know that you ovulated 2 weeks previously.
but whenever you'll ovulate next is not necessary 14-15 days!
you might ovulate again from the other ovary while you are having a period or soon after.
or even between first ovulation (say Day 10) and a period (say Day 24), in which case you could get pregnant in that cycle on say day 18 and be actually 1 week pregnant while you are having a period!!!!!
or if you are ill or stressed or have a trauma etc your ovaries won't release any eggs to stop you from conceiving at a bad time so to speak in that case yo u could ovulate days or weeks later than you should be or just skip one...
if you ovulate late in your cycle, say Day 27 and get pg, the test won't even show anything until about day 44 or so!
in which case it's even more confusing, because the embryo/foetus is only 2 weeks old, but you are 6 weeks pregnant!
which is what happened to me with DS2!!!!
does that make sense?