Pregnancy Timeline: Determining Conception Date & Gestation Weeks

Question
I had my full and normal 5 day period on oct.1, 2007.  Then on nov.1, 2007 i had only one day of bleeding and it stopped.  I did home pregnancy test two weeks later on nov.16, 2007 (to make sure) and sure enough it came positive.  Blood test showed 6 weeks and ultrasound showed 8 weeks. My question is when did i really get pregnant and how many weeks pregnant am i right now?  My doctor counts weeks starting november as my lmp, but i think he is wrong and i am really 3 weeks ahead in my pregnancy.  I also feel alot of rectal and pelvic pressure, nausea and ocassional diarreah.  Does this mean i can go into labor soon...?

Answer
If an ultrasound showed you to be 8 weeks, you should definitely be using that due date.  If it is earlier than the due date your doctor is using, then you very well may deliver "early" by his clock, but your baby would be perfectly fine.  You may want to bring this up with your doctor and find out why exactly he is not using the ultrasound dating, especially since it agrees with your own symptom history.  However, there isn't much danger involved here; your doctor has a later due date than you do, so you're not going to get pressured into an induction for being late.  At worst, your doctor may want to stop labor if it starts "early", but you can always refuse if you are past 37 weeks by the ultrasound dating.