I, too, have never heard of anything related to dyslexia and sonograms/ultrasounds. That's a new one on me. So I did a web search.
After searching several permutations of "sonogram +dyslexia" and "ultrasound +dyslexia" I was only able to find one article from 1992 that seemed to have any credibility.
The American Institute of Songraphers (or whatever the group is called) doesn't recommend non-medical use of sonograms, but they do recommend them for medical reasons.
I have absolutely no qualms about having ultrasounds/songrams during my pregnancy... I hope you end up feeling the same.