U/S Dating Question

APrice

Okay so I posted yesterday about my first u/s and how the doctor was worried about the heart rate being low for how far along I am (at 75 BPM). So I woke up this morning spotting a little bit and called as my doctor had instructed me to. The nurse calls me back and she tells me not to worry too much unless the spotting is bright red or I have severe stabbing pain/cramps. She also mentions that the u/s I had can cause slight spotting. While I have her on the phone she mentions that the u/s tech dated my pregnancy at 6 weeks and 2 days (based on measurements of gest. sac and yolk sac.) and I explain to her that this is virtually impossible because my LMP was 9/18 and I charted so I know I ovulated on CD 14/October 1st. What do you ladies think? Is it possible to be measuring further along but not really be? This may explain why the doctor was worried about the heart rate....my mind is racing over this! 

Sorry that was so long and wordy...just trying to give you as much info as possible. Thanks in advance!