I completely agree with thorn that the decision regarding whether or not to obtain any prenatal ultrasound is entirely personal and up to the mother and family.
And it's true that ultrasound has never been proven to cause growth retardation and cell mutation in fetuses. However, it has been correlated with these two issues, among many, many, many more.
Ultrasound has never been proven to be safe. (I recommend everyone do thorough research [Note: this little blog post is obviously biased, but has links to dozens of studies/research/articles regarding the potential dangers of ultrasound] before making a decision.) I'm, personally, not willing to take any chances with my baby at this moment in my pregnancy. I am always open to changing my mind if at any point I feel an ultrasound to be medically necessary. Again, this decision is just so personal. Parents have to weigh the risks vs. the benefits of prenatal ultrasound for their family and during this particular pregnancy. There is no right or wrong INFORMED decision.