OMG.. i never thought of it in that way before! 7 weeks... wow..
Is it really that common for first time moms to go over there due date? (even if i know my exact ovulation date, i was BBTing and using OPK i also get mittelschmerz.)
It is much more likely to go overdue with your first. Obviously, not all women will go overdue with their first, but the chance is very high. Also, you can certainly go overdue with subsequent babies (of course theres always a chance any pregnant lady can go early or overdue no matter which number baby) According to Dr. Bradley, the average first time mother will go one week and one day "overdue" (i believe if the EDD is based on LMP). With second (or more) mothers, you are statistically likely to go 3 days overdue (however you are significantly more likely to go before your EDD than with your first). just remember these are averages, so it varies for everyone.
Even if you know when you ov.ed it doesn't mean anything. Even if you had an early scan-doesn't mean anything. And even if you have your cycles down to a T and everything is textbook-it means ZILCH. Basically the baby decides no matter what.
For instance, based on LMP Isaiah's EDD was Aug. 23, 2010. Based on early ultrasound it was August 25th, 2010 and based on ovultation (i was tracking on fertilityfriend and i am CERTAIN of when i ov.ed), i was due August 26th, 2010....well they kept my EDD at August 23 because their policy was to only change it if there was a 10 day difference (which is a dumb policy IMO, especially with how pushy US docs are to induce

) Anyways, i didn't go into labor until 41+4 and i had him 30 hours later at 41+5. (based on LMP EDD)
So...If LMP were based on:
LMP, I was 12 days "overdue" (1 week & 5 days "over")
Early US, I was 10 days "overdue" (1 week & 3 days "over")
Ovulation, I was 9 days "overdue" (1 week & 2 days "over")
As you can see, it doesn't matter if you know the "exacts" of conception (or cycles or anything else), every baby (even in the same mother) has different things going on with their development. The lungs are the last thing to develop. Once they are developed sufficant is realeased and that triggers labor. Some babies will develop the sufficant earlier-some later. It really doesn't have much bearing in reguards to conception date. Obviously it will be close, but its generally not an exact science since each baby develops at different rates.