If your know your LMP and your OB office doesn't usually do a dating ultrasound, is there anything I can say to maybe convince them to give me one?

I know I ovulated later than CD 14, but I'm not sure saying "I have long cycles, I'm not sure when I ovulated" will be enough to convince them.

Were you able to get a dating ultrasound? What'd you say/do to have that happen? Any advice for me?