paying for a gender US and they want old US record?

Rachael
I called to set up an US at a clinic two hours away because unfortunately we don't have a place closer that will let you pay for the gender scan. At first they weren't even sure they would do it with me being 18 weeks and then they asked when my last US was. I told them 13 weeks and they said they would only do it if I brought in that 13 week US report. I don't mind at all and Ofcourse agreed but does anyone know why? I'm kind of embarrassed to go into my OB and ask them for that report because I'm too impatient to wait another three weeks to find out lol. Again I'm just super excited to find out Friday I was just curious as to why they want that(: