baby measuring small..

Hi ladies, so my first ultrasound was on Thursday (7/26) and based on my LMP, I should have been 6 weeks and 6 days along. My baby is measuring at less than 5 weeks and 5 days with a heart rate of 70 bpm (I forgot to mention in my original post that my HCG on 7/16 was 871). I know for sure that I ovulated later than I typically do because my progesterone didn't peak until several days after CD 21. I'm worried because I'm thinking it won't be viable. Has anyone experienced this? What was your outcome? Thank you for any responses