Okay so I am pregnant with my 4th child and I have never understood how they calculate how far along you are. It is not possible to be pregnant on the first day of your period so to count day 1 as pregnancy day 1 seems stupid to me. Now my due date and the Dr.'s due date are only different by a day so it shouldn't matter much except that by putting me 2 weeks ahead of my pregnancy by going from my period would put baby 2 weeks ahead in testing and ultrasound. 2 weeks of a difference is a big deal for a developing baby. Apperently knowing conception date is an oddity but yes I know exactly when I concieved. We were trying, we didn't have sex for days and days prior to ovulation, I have regular cycles and ovulate 14 days before my period (I confirmed this with an ovulation test) we did not have sex for days after ovulation (when ovulation test said I was no longer ovulating. Given sperm living 5 days but egg only living 24hrs conception had to occur within that 24hr period. So why, given all of this, would I be 2 weeks further along then I actualy am? A pregnancy is only 38 weeks but due to the Dr.'s putting you 2 weeks further, they go by 40 weeks so the due date is the same. I do not understand why though, they would put you 2 weeks ahead if you know conception date? At first ultrasound they are expecting to see an 8 week fetus but its a 6 week fetus would that not throw the testing and measuring off?