unsure of due date

Stef1018

i took a pregnancy test exactly 2 weeks ago and it was positive. i couldn't remember though when my last period was so i guessed when my DR asked. based off my guess i should have been about 6 weeks pregnant but they didn't see anything on the ultrasound (except a yolk sac). they confirmed my pregnancy with a positive test but i'm just really nervous that when i go in there on friday (ten days after my last appt) there isn't going to be anything in there...