Thought it may be helpful to share my experience.. at my 6w6day transvaginal ultrasound, my ob told me I looked like I was one week behind than expected . He did not seem concerned and did not adjust my EDD then but of course I was so worried . I have a very regular cycle and used opks ( but of course no one really knows when egg and sperm in the end ). I just went back two weeks later for my “8 week 6 day “ abdominal ultrasound “ n I was only dating 1 day behind so now I’m 8 weeks 5 days. When I asked him he said oh before fetus is 8 weeks old it is highly inaccurate to date.. he has now adjusted my EDD to just one day behind . Phew! Hope this helps some other worrying mothers to be .