More ultrasounds??

When I first found out I was pregnant I went to a clinic in my area and they gave me a sonogram when I was 8 weeks along. Of course there wasn't much to see besides a little jumping bean but I really enjoyed it. My next sonogram was when I was 18 weeks and it kinda wasn't super clear. My doctor believes I am farther along than I am and it just wasn't a good experience. I really want another sonogram because I don't feel like I got the full effect with either one. How can I go about this at my doctors office? My insurance covers 2 sonograms at my doctors office but when I brought it up he said that already had 2. I told him that I had one at the clinic and one at the doctors office and he kinda just brushed it off. Advice please? Thank you!!!