You can probably ask about one but I don't think they'll do one yet. As early as you are the babies are so very tiny it's hard to see anything on ultrasound and some moms really freak out because it looks like there's no baby, only to find out that everything is fine later on.
Thank you.. at least now I know
Try for an ultrasound in your 7th week. They might not be willing to do it unless you have a compelling reason, i.e., you don't know which of two guys is the father, or something like that. A lot of insurance coverage doesn't pay for "just out of curiosity" ultrasounds. But if you do get one for purposes of dating the pregnancy, the 6th or 7th week (counting from the first day of your last period, not from conception) is the time to do it.
My doctor's office won't do a dating ultrasound until the eighth wk.