40 Weeks???

Nine months ≠ 40 weeks.  Last time I checked, that would be 10 months.  What’s up with that?  Can someone please explain this discrepancy to me? Would a nine-month pregnancy baby be considered a preemie?  How long was your first pregnancy?  Was he/she considered early? Late? Right on the due date?  I have heard that most 1st babies come early.  I have also heard that most 1st babies come late.  Please help shed a little light on the subject.  It just doesn’t seem to add up to me.