10 days til my dating ultrasound

This is my first pregnancy. I believe I am about 5 weeks along. Had to take clomid to assist with ovulation, and am used to 45+ long cycles. I didn't ovulate until 21 days on clomid, so I believe my date is somewhat off. I am so excited & afraid for my first ultrasound. I'm fearful that my doc will tell me that I won't have a viable pregnancy because I am confident that it will measure much smaller than the doc believes it should be.