rounding errors

one of my students cancelled, so here I am blogging instead. but since I'm also thinking about math, let me share some math with you.

from a nickelodeon / parents connect / 3d pregnancy site on the "8 weeks" page:
Your growing baby is six weeks old, thanks to the fuzzy math of pregnancy. This fuzzy math is also responsible for the widely believed notion that pregnancy is only nine months long, when in actuality it's closer to 10. Who comes up with this stuff? Obviously a man.

I disagree - or at least I don't understand....
here's how I think people come up with the idea that it's 10 months: 40 weeks, about 4 weeks to a month, 10 months.
but here's what I get when there's no rounding... 40 weeks... but wait, you're not pregnant for the first two weeks - they're freebee weeks you just get if you count from the first day of the mom's last period instead of counting from conception (which is why above they're saying your baby is only actually 6 weeks old, not 8). so really, you're only pregnant for 38 weeks.
38 weeks * 7 days = 266 days.
now let's say there are 30.5 days to a month, 9 months would then equal... 274.5 days, or 8.5 days longer than you're actually pregnant for if you make it to your "40 week" due date.
even if you don't subtract those freebe weeks, you still get 40*7=280 days. meaning 9 months and 5.5 days. which is still closer to 9 months than it is 10 months.
am I missing something?

all that said... I plan to count my first trimester as over when I get to week 12 (which definitely includes the fuzzy math and rounding down) because well, I want to. and I think I'll still be wishing it was even shorter by the end... :)

next kid due in 2 minutes (tutee, not baby). later gators.