Question about early ultrasound

I went for my first OB appt. yesterday and had an ultrasound - I was supposed to be 7 weeks, 4 days pg. The ultrasound measured the fetus to be around 6 weeks, with a normal heartbeat. My dr. seemes very skeptical since it is not possible that I have my dates mixed up - I KNOW I did not ovulate late because I got such an early bfp 4-5 days before AF was due. Anyways, I have to go back in one week to see if there was any growth, but I wondered if anyone has had a similar experience, and what was the outcome? Can a fetus just be "small" this early? It is hard to have to wait a week - I am so nervous. Thanks.

Re: Question about early ultrasound

the dr's won't worry unless the measurement is more than 2 weeks difference. For example, I went for an U/S when I was cramping/spotting around 10 weeks and the sac was still only measuring around 7 weeks, when we had the first sono. That's when they knew that it was not a viable pregnancy.

Don't worry about it!