There was a study done awhile ago, and reviewed in a parenting magazine, that said the old wive's tale (about women getting heartburn will have children with hair) is actually true. They discovered that the hormone that grows baby hair in utero also often gives the mother heartburn. I found it to be true for me. I had horrible heartburn with my first three children and they all had lots of hair.
I hope this helps. I thought it was an interesting study. Here's a link:
http://www.associatedcontent.com/article…
ETA: As far as the ultrasound, I think it depends on the expertise of the ultrasound tech and how much hair the baby has at the time of the ultrasound. Also, the type of ultrasound done. For instance, a 3d or 4d ultrasound would probably make it easier to see hair than a 2d.