math challenged

Something's been bothering me for 31 1/2 weeks: if a pregnancy lasts nine months, why is my due date 40 weeks? I feel too embarrassed to ask my doc (or anyone else for that matter), but no matter how I slice it I can't get the numbers to work.
So today I am reading, The Must-Have Mom Manual, and here is a quote from pg. 27 . . .
"Dads, even though you are in the labor room, you should still expect a lot of waiting. Movies portray a huge rush to the hospital: throwing the suitcase in the car and speeding through streets in the family car (which is probably the only part most men look forward to - a reason to speed legally) while the mother is breathing hard, sweating, and cursing. Upon arriving at the hospital, nurses scurry the expectant mom into the delivery room and immediately people start screaming, 'Push!'
Ah, that's not at all how it happens. And we'll never understand why the media seem to consistently portray it incorrectly. (Just like that whole thing about being pregnant for nine months. Nobody's pregnant for nine months. It's forty weeks, and that means ten months, people!)"
Here's more on the subject . . . A pregnancy is suppose to last 40 weeks but not really because they add the 2 weeks between your LMP and your conception date. It's weird, so actually it's 38 weeks. P.S. Never count the months... counting by the week is way more accurate.
Source(s):
Pregnant for the 4th time.
And more . . . Gestation in a human being is 266 days. (9 months is actually 274 days; 40 weeks = 280 days.). The reason that most doctors say 40 weeks, even though that is not biologically correct, is that they start the count at the first day of your last period. That adds 10- 15 days to the actual number of days of gestation. From the day of actual conception to birth is 266 days. For your own information: The actual length of any animal's gestation has to do with size. The larger the animal, the longer the gestation period, in general. Be glad your not an elephant! Indian Elephant gestation= 624 days.
I wonder if this would be any easier in metric?