help me understand measuring behind

Hey if anyone could help me understand measuring behind that would be great because I'm a little worried. From my lmp I should be 5w1d, I had an early scan today and measured 4w5d. I had a positive ovulation test on cd 18 and had my first positive on oct 24. On oct 24 I would have been 2w6days according to the scan today. Does this seem right or does it seem like something could be wrong with my baby?