@beachbum44 , if it concerns you, I would simply call to cancel next week's ultrasound. And let your OB know your concerns at your next appointment, and explain you would like to do just one more ultrasound unless there is an actual development of your personal circumstances (like going over 42 weeks, or heavy bleeding) that would warrant another look. If you are feeling like you don't need all this, you need to be able to tell your care provider that and have your decisions respected. I wish I had just said, "I said no, so lets move on to the next topic." Dwell on it and medical professionals tend to think that you still want more advice, when really, you are good on this one.
Personally I feel two ultrasounds per baby is a fair number. More is too risky for me to do without careful consideration and good reasons (I did have three with my last baby, and she is fine). We are going to do the 20 week and try to keep it to that (unless, of course, we need it for outstanding or unusual reasons like I said above, we had that with the last baby because of all the different due dates and sizes and bs. We got dates from 11/24 to 12/12 and had a thrid U/S to get a "final" due date so that we didn't worry about her being too early or late for the home birth. It sounds like you are going to a hospital, so early or late won't really matter with all the OB's on hand).