.O.K. first of all...I'm not carrying a little one right now. Had my little bundle of joy back in September 2007.
I'm on my way to becoming a Sonographer (attempting anyway) so, I thought that I would chime in.
I had a miscarriage in 1996 and a full term stillbirth in April 2006. So, with my little boy I had about 8 or so ultrasounds. I was considered high risk and there were some complications with the pregnancy.
I'm not a Sonographer, but I do know a little something about radiation. Ultrasound uses thousands of soundwaves/sec, non ionizing radiation which creates an image. The term radiation is used because, like most things it radiates. When you go outside, you get hit with more ionizing radiation (the bad stuff) than when you get an ultrasound. The higher in altitude you are, the more cosmic radiation (ionizing) you receive. When you fly, you're also getting hit with the bad stuff. Keep in mind though, I think you have to fly between N.Y.C and Athens roundtrip about 12 times in a year for it to be a bit of concern to the little one.
I'm not saying that I know of all of the long term effects on babes in utero, how could I? The technology has only been around since the 60's. X-Ray has been around since 1895, so we know that a lot of it, without the right precautions is not good. Heat from the transducer seems minimal. When I have an ultrasound, you know what I'm mostly concerned about? How hard the Sonographer pressing. Technique is what everyone should be concerned about, you have a bad sonographer...do not go back -- doesn't seem worth the risk. (Not sure if there is any, I was just taught that in anything medical, someone with bad technique is not someone you want to go back to.)
I can tell you that ultrasound has taken the place of some x-ray procedures. An ultrasound can view the heart, the liver, kidneys, and little sweet babies without the use of any invasive procedure and no barium (yuck, who wants to drink that stuff).
I'm not saying that everyone should go out and get one tomorrow. You have to do what's right for you and only you can determine that. Just thought that if radiation was a concern for mamas...I could shed some light.